Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Extracellular matrix wikipedia , lookup
Tissue engineering wikipedia , lookup
Cell growth wikipedia , lookup
Cell encapsulation wikipedia , lookup
Cellular differentiation wikipedia , lookup
Cell culture wikipedia , lookup
Organ-on-a-chip wikipedia , lookup
List of types of proteins wikipedia , lookup
AMER ZOOL. 16:277-291 (1976) Spiralian Development: A Perspective DONALD P. COSTELLO AND CATHERINE HENLEY Department ofZoology, University of North Carolina, Chapel Hill, North Carolina 27514 s\ \ORSIS Three basic t\pes of spiral cleavage are described: spiral cleavage by quartets, by duets, and b\ monets. Following F R. Lillie's concept of adaptation in cleavage, the relation of each of these cleavage modifications to the structure of the lar\a or juvenile, which develops therefrom, is considered. These comparisons lead to the conclusion that Lillie's concept of adaptation in cleavage can be extended beyond such details as cell contents, cell size, tempo of diwsion, etc., to include even the oblique character of spiral cleavage and its general basic form. INTRODUCTION Seventy-five years ago, one of the most eminent students of the emerging field of cell lineage wrote the following prescient words: "It has happened, very naturally, that writers on the subject of cell-lineage have laid special emphasis on the resemblances, which are nothing short of marvelous, . . . between the cleavage of the eggs of even widely separated forms. . . . But . .. one of the most instructive aspects of cell-lineage is thus lost sight of, namely, the special features of the cleavage in each species, which are, 1 believe, as definitely adapted to the needs of the future larva as is the latter to the actual conditions of its environment" (Lillie, 1899; p. 43). In this perspective, we propose to examine Lillie's concept more closely, and to relate it to modified forms of spiral cleavage as it occurs in several groups. Before we do so, however, it is appropriate to consider briefly what is meant by spiral cleavage and, interestingly, the very origin of the descriptive term "Spiralia." "SPIRALIA" AND SPIRAL CLEAVAGE Schleip, in 1929, defined Spiralia as those animal groups showing spiral cleavage during early development. However, since the primary basis for setting up phyla is comAided by a grant from the National Institutes of Health, GM 15311. We thank Miss Barbara G. Cain for her efficient assistance with the illustrations. parative anatomy rather than comparative embryology, the term "Spiralia" has no obvious supraphyletic significance. The difficulty arises from the fact that all representatives of the phyla involved do not show spiral cleavage. It is generally agreed that the forms showing clear-cut spiral cleavage by quartets include the polyclad turbellarians, the annelids, the rhynchocoels, the molluscs (with the notable exception of the cephalopods), the echiuroids, and the sipunculoids. Modified forms of spiral cleavage are characteristic of the acoel turbellarians, of at least one group of arthropods (the cirri pedes) and of the rotifers. The view that cleavage homologies are a criterion of phylogenetic affinities dates back to the earliest studies of spiral patterns of annelids and molluscs, to which that of the polyclad turbellarians was soon added. There was the general idea that a primitive coelomate ancestor, with spiral cleavage of the quartet type, gave rise to a turbellarian archetype, from which higher forms were evolved. This was discussed as early as 1914 by Heider, although we do not find the term "Spiralia" in Heider's paper. Heider's ideas had been strongly influenced by Hatschek's (1886) trochophore theory, which stressed the affinities between the annelids and molluscs, as indicated by features common to their larval forms. Aside from the chapter heading in Schleip's (1929) book, the chief expositions that we have found dealing with the "Spiralia" are by Ulrich (1951), Remane (1952, 1957, 277 278 DONALD P. COSTELLO AND CATHERINE HENLEY 1963) and Marcus (1958). Marcus speaks of more thorough investigation is completed. Remane, Ulrich and himself as being in It is clear that Blochmann (1881) used a "common zoological descent from Karl fully developed system of capital and lower Heider," in relation to their views on the case letters, plus numbers and exponents, evolution of animal phyla. Peter Ax (1963) to describe the spiral cleavage pattern of stated that in his view the spiralian theory the developing embryo of Neretina. Fol was clearly the most probable interpreta- (1875) had earlier employed a system based tion of the phylogenetic position of the on the Roman numerals I-IV for the four Turbellaria. macromeres and the Arabic numerals 1-4 Spiral cleavage is characterized by a rota- for the first micromeres, in his description tional movement of cell parts around the of pteropod cleavage. Whitman (1878) egg axis, leading to an inclination of the who, perhaps more than any other invesdivision spindles with respect to tigator, may be said to have founded cell symmetrically-disposed radii. It could, lineage, used the letters a, b, c, and x for the more properly, have been called oblique four basic blastomeres of Clepsine. Wilson cleavage. These rotational movements fol- (1892) and Conklin, prior to 1897, had delow a regular alternation of direction, vised usable, slightly dissimilar systems for clockwise (right-handed, or dexiotropic) Nereis and Crepidula, along lines akin to the and counterclockwise (left-handed or laeo- terminology of Blochmann. Robert (1904) tropic [ = leiotropic or laevotropic]), in suc- improved Conklin's scheme by identifying cessive cleavages. Post-cleavage rotations of the macromeres with capitals and prefix cell parts and blastomeres may occur also. numbers to indicate their generation The inclination of the spindles in spiral (1A-1D etc.). Meanwhile, Kofoid (1894, cleavage of the quartet type is usually strik- 1895) had devised a new scheme by which ingly clear at the third cleavage, at which each cell during the spiral period of cleavtime the first quartet of micromeres is age was designated by (1) a lower case letter formed. During the first and second cleav- (a, b, c or d) indicating the quadrant, (2) a ages, which may be either equal (Fig. 1) or first exponent indicating the cell generaunequal (Fig. 2), the obliquity of the spin- tion, and (3) a second exponent to desigdles is less apparent, and the chief criteria nate the quartet (or "story"). Kofoid, howare the pre- and post-cleavage rotational ever, numbered the quartets from the vegmovements. These first two cleavages may etal pole toward the animal pole, and desbe spoken of, therefore, as prospectively ignated the lower cell or quartet by an odd spiral (Conklin, 1897). Except in those exponent and the upper by an even one. forms showing reversed symmetry, it is the Thus, his second exponents are directionodd-numbered divisions that are dexio- ally reversed as compared with the corretropic, and the even-numbered laeotropic. sponding designations for quartet and cell In spirally cleaving forms, there is a transi- positions of the other schemes. Kofoid's tion to bilateral divisions following the early nomenclature was avidly applied in segmentation of the embryo, and this fact numerous molluscan and annelidan cleavhas considerable significance in relation to age studies, and modified only slightly for Lillie's ideas about adaptation in cleavage. cleavage in the cirripede egg by Bigelow (1902). Anyone familiar with invertebrate We feel that if cleavage patterns are to be cleavage patterns can readily translate any used as criteria of embryological relation- of these terminologies into any other (see ships, both the obliquity of the spindle orienta- Fig. 6), but it is essential to keep in mind the tions and the regular directional alternations of exact meanings of the designations in the spiral cleavage must be stressed. different systems.1 This translation is as TERMINOLOGY OF SPIRAL CLEAVAGE The origin of the four-quadrant system of cleavage nomenclature is shrouded in obscurity and is likely to remain so until a 1 In our estimation, Conklin's adaptation of Kofoid's scheme for the ascidian egg is a perfect or near-perfect nomenclature for that group. For the Molluscs (and Annelida), we favor Robert's system. SPIRALIAN DEVELOPMENT: A PERSPECTIVE simple as converting fractions to decimals. Obviously, all four-letter systems of cell nomenclature were devised for the type of spiral cleavage in which there are four clearly discernible quadrants, marked (usually) by four large, yolk-filled macromeres, which produce successive quartets of micromeres. \nNereis, Wilson (1892) observed that the oil droplets (as well as the yolk spherules) accumulate in the macromeres and gradually fuse to form a single large oil drop in each of the four large endodermal cells, to provide a striking visual criterion of the quadrant arrangement. FOUR-QUADRANT SPIRAL CLEAVAGE The classical example of molluscan four-quadrant spiral cleavage is that of Crepidula, as described by Conklin (1897). As shown in Figure 1, the first two cleavages are equal and meridional, giving the four cells A, B, C and D, with a vegetal polar furrow resulting from the early rotational protoplasmic movements. It is the third 279 cleavage which is clearly spiral. The spindles in each of the first four blastomeres become oriented obliquely (and dexiotropically) to produce the first quartet of micromeres (la-Id) nearer the animal pole and the four yolk-laden first generation macromeres (1A-1D) at the vegetal pole. In subsequent successive cleavage cycles two additional complete quartets of micromeres (2a-2d and 3a-3d) are separated from second generation macromeres (2A2D) and third generation macromeres (3A-3D). These divisions lead to the 12-ceII and 20-cell stages and are, respectively, laeotropic and dexiotropic. At the 24-cell stage a micromere spindle appears in the macromere of only one quadrant. The ensuing laeotropic division separates 4d (the mesentoblast) from 4D, identifying the left posterior macromere. Soon after the formation of 4d it divides dexiotropically into right and left halves. These cells lie to the right of the future median plane, which is marked by the second cleavage furrow. These soon divide again, cutting off a pair IB la 1 2b la 1 3a 3B 3C FIG. 1. Cleavage of Crepidula (redrawn from Conklin, spindles for 2nd quartet with 1st quartet cells rotated 1897). Top row, left to right: Two-cell resting stage, polarinto macromere furrows. Bottom row, left to nght: De\ioview, showing dexiotropic position of nuclei and asters tropic spindles for 3rd quartet, with central cells, lurprior to their laeotropic rotation; four-cell stage, with ret cells and 2nd quartet constituting cap; 24-cell stage. nearly radial spindles, prior to their dexiotropic rota- with spindle for 4d (mesentoblast): side view. 29-cell tion for the third cleavage; laeotropic fourth cleavage stage, showing relation of 4d to 4D. 280 DONALD P. COSTELLO AND CATHERINE HENLEY of primary enteroblasts posteriorly. The first bilateral cleavages thus appear suddenly at the 42-cell stage. It is not until the 68-cell stage that the mesoblasts are completely separated from the enteroblasts. Spiral cleavage of the quartet type does not necessarily involve two initial equal divisions (Treadwell, 1899). In Nereis (described initially by Wilson, 1892, and with slight modifications of terminology by Costello, 1945) the first two cleavages are unequal (Fig. 2). Thus one can identify the four quadrants as early as the 4-cell-stage. Aside from this inequality and the fact that the quartets of micromeres are considerably larger in Nereis than in Crepidula, the basic patterns of spiral cleavage of these forms are essentially similar. Transition to bilateral cleavages begins at the 38-cell stage in Nereis. At this time the primary mesoblast has just been segregated. The parallel fates of the cells after the segregation of ectoblast, mesoblast and entoblast, in the production of trochophore or veliger larvae, have been described by a number of authors for the various annelidan and molluscan species, and will not be repeated here. Obviously, there are differences, also, especially in later larval development and during metamorphosis. THREE DEPARTURES FROM THE FOUR-QUADRANT SYSTEM OF SPIRAL CLEAVAGE Acoel turbellarians A simpler type of spiral cleavage, characteristic of the acoel turbellarians was described by Gardiner in 1895; it involves the formation of only two large macromeres and successive pairs of micromeres. A suitable system of nomenclature, involving halves instead of quadrants, and duets of micromeres instead of quartets, was devised by Bresslau (1909) and employed by 2d FIG. 2. Cleavage of Nereis (after Wilson, 1892, as modified by Costello, 1945). Top row, left to right: Two-cell stage, polar view; 4-cell stage, polar view, with vegetal polar furrow indicated; 8-cell stage, with 1st quartet of micromeres in dexiotropic position above macromeres. Bottom row, left to right: 16-cell stage, showing central cells, trochoblasts, 2nd quartet and macromeres; 16-cell stage, from right side; 22-cell stage, polar view, with 2d about to divide and formation of the two posterior 3d micromeres. (re-used with permission of the Wistar Press) SPIRALIAN DEVELOPMENT: A PERSPECTIVE 281 that author (1933) and by Costello (1937, tion. Subsequent cleavages are not clearly 1961). spiral or oblique in orientation and, like those in four-quadrant spirally cleaving Essentially, the process involves a meridional first cleavage, producing two blasto- eggs, undergo a transition to bilateral divimeres equal in size. The second division is sions, foreshadowing bilaterality in the not meridional, as in four-quadrant spiral juvenile. cleavage, but rather is oblique, cutting off A clue to the possible role of centrioles in two micromeres (la and lb) at the animal orienting the alternately laeotropic and pole; it is laeotropic and unequal (Fig. 3). dexiotropic spindles of spiral cleavage by The first duet of micromeres likewise di- duets in the acoel Polychoerus was offered by vides obliquely, in a dexiotropic division, to Costello (196la). In this form the 2n (34) produce la 1 and la 2 , and lb 1 and lb 2 . chromosomes are arranged in a circle at About this time the two first generation metaphase around the central spindle of macromeres cleave, in an orientation which the equal first cleavage. At each pole there is dexiotropic, but tending toward the bilat- is a remarkably large (4.5-5 /im long) roderal condition, resulting in a pair of second shaped centriole, readily visible by light duet micromeres (2a and 2b) and a pair of microscopy. The axes of the two centrioles second generation macromeres (2A and are at right angles to one another, and to 2B). At the 8-cell stage, a third duet of the axis of the spindle. Subsequent to the micromeres (3a and 3b) is produced by a publication of that paper, further study of cleavage which, again, is laeotropic, with cleavage spindles by light microscopy resome tendency towards the bilateral condi- vealed that replication of these long cen- FIG. 3. Cleavage of Polychoerus carmelensis (redrawncells, after division of 1st duet; 8-cell stage, after forfrom Costello, 1961). Top row, left to right- Two-cell mation of 2nd duet. stage; 4-cell stage, side view. Bottom row, left to right: 6 282 DONALD P. COSTELLO AND CATHERINE HENLEY trioles occurred at their centers, not near the ends. If daughter centrioles formed by such a pattern of replication separated from their mother centrioles, obliquely and at right angles in the A blastomere as compared with the B blastomere, the second cleavage spindles thus generated would be in the precise laeotropic orientation which has been observed. A similar pattern of centriole replication in the upper cells at the third cleavage would result in dexiotropically oblique spindles. Thus, in this case at least, the regular alternation of laeotropic and dexiotrophic divisions for the early divisions can be explained by the mode of replication of the centrioles, so long as the progression is toward the animal pole. There is no apparent a priori reason why the same, or a similar, mechanism could not be the clue to spiral cleavage of the quartet and monet types as well. As a matter of historical interest, Figure 4 shows some sketches of the development of the acoel Polychoerus caudatus, made at Woods Hole in 1890 by Thomas Hunt Morgan. They are probably the earliest camera lucida sketches ever made of duettype spiral cleavage and were never published. Morgan's hand-written comments accentuate the excitement attendant on new discoveries at that time. The accuracy of his observations can be attested by studies made in 1968 by Costello (also unpublished) of cleavage stages of living eggs of the California species of Polychoerus. Bresslau (1933) established the eventual fates of the cells in acoel embryos, and showed that the first duet of micromeres produce ectodermal structures, including the nervous system. The second and (probably) third duets of micromeres form epidermis and peripheral parenchyma, while the fourth duet of micromeres gives rise to part of the internal mass of the embryo (which, of course, lacks a defined gut). The fourth generation macromeres likewise contribute to this internal parenchymal mass. Bresslau's observations have \r U. (1 1 FIG. 4. Four of Morgan's 1890 sketches, showing a sequence of cleavage divisions in a living embryo of Polychoerus caudatus. SPIRALIAN DEVELOPMENT: A PERSPECTIVE been confirmed and extended for a number of other acoel forms by Apelt (1969). In the polydad turbellarians, which have the typical four-quadrant pattern, the four quartets of micromeres have fates similar to those described above for the duets of acoels: the first quartet produces anterior dorsal ectoderm, eye pigmentation, and ganglia of the nervous system; the second quartet cells likewise contribute to the dorsal epidermis, and in addition their progeny produce mesectoderm, forming muscular and mesenchymal portions of the pharynx. The fate of third quartet cells and their descendants is similar to that of the second quartet. Three of the fourth quartet micromeres (4a-4c) do not divide further but are nutritive; the fourth member of this quartet (4d) eventually gives rise to mesoderm and endoderm and, as Kato (1940, 1957 [trans. 1968]) points out, is thus entirely comparable to the mesentoblast cell of the molluscan embryo. It is important to emphasize here that the acoels have direct development, with no intervening larval stage, and hatch as miniature bilaterally symmetrical adults. The polyclads, in contrast, often produce larval forms which have at least a superficial resemblance to the trochophores of annelids and molluscs (see, also, Henley, 1974). 283 which the yolk is still contained. The four cells were described by Bigelow as becoming "adjusted in a laeotropic arrangement," and he points out that even in this early stage there is a suggestion of coincidence between this arrangement of the cells and the future sagittal plane of the embryo. The third cleavage is "equatorial," and again the micromeres divide equally and synchronously, while the macromere divides unequally and is slightly retarded, the yolk again being segregated into the macromere daughter cell (3D), while the micromere daughter (3d) contains ectoblast. Bilaterality in cleavage is well established by this stage. The primary mesoblast cell (4d) is segregated from the yolk-laden macromere (4D) at the fourth division, so that at the 16-cell stage the entoblast is completely separated from the other germ layers. Bigelow describes the origin of mesoblast as being dual in nature, part derived from entoblast at the fourth cleavage and part from ectoblast at the sixth cleavage. The four secondary mesoblast cells derived from the sixth cleavage apparently form "at least the mesenchyme of the Nauplius." This includes the musculature of the paired larval appendages. The blastoderm, Bigelow emphasizes, is formed from derivatives of three, and only three, micromeres. It is, of course, the first three single micromeres (Id, 2d and 3d) that give rise to ectoblast and, later, to the secondary mesoCirripedes blast. The single fourth micromere of Bigelow's description is primary mesoblast, A second major departure from and he refers to the precocious segregation quartet-quadrant type spiral cleavage was of ectoblast beginning without the previous found to occur in the cirripede crustacean division of the entoblast into a quartet of Lepas, as described by Bigelow (1902). The cells. "As a result of this there \s in Lepas one first cleavage of this egg is unequal and entoblastic macromere instead of four, as in markedly oblique (Fig. 5), dividing the zy- annelids and molluscs, and single microgote into a smaller anterior cell, mostly meres appear to represent quartets. So far protoplasmic (the first micromere, Id by as the order of cleavage involved in the our terminology), and a larger, yolk-laden segregation of the primary germ layers is posterior cell, the macromere (ID). Next concerned, the first micromere of Lepas apthere is an almost simultaneous "merid- parently corresponds to the first quartet of ional" division of each of the two blasto- ectoblastic micromeres seen in the eightmeres, which is perpendicular to the first cell stage of such eggs as have four macrocleavage plane; the micromere (Id) divides meres resulting from the [quadrant-formequally, giving rise to two daughters (Id 1 ing] (first and second) cleavages. The and Id2), while the macromere (ID) has an micromeres of Lepas are, then, according to unequal division, producing a smaller this view, to be regarded as equivalent to micromere (2d) and a macromere (2D) in id id FIG. 5. Cleavage of Lepas (redrawn from Bigelow, anaphase, second cleavage, viewed from above animal 1902, but with our designations). Top row, left to right- pole, the oblique line designating the sagittal plane of Zygote with polar bodies (2nd polar body attached.at the later embryo. Second row, left to right: 8-cell stage animal pole) and yolk in an eccentric position at the from above animal pole; same, from vegetal pole; 16vegetal pole; first cleavage anaphase in egg under- cell stage from animal pole; same from vegetal pole. going typical rotation; two cell stage, with clear Third row, left to right: Parasagittal section of late 16-cell micromere and yolk-containing macromere; late stage; same of later cleavage stage, with primary SPIRALIAN DEVELOPMENT: A PERSPECTIVE the quartets of micromeres, while the single yolk-macromere equals a quartet of macromeres" (Bigelow, 1902; pp. 124125). Considering Bigelow's acuity in recognizing the significance of the cell homologies of Lepas, when compared with those of many annelids and most molluscs, it is indeed unfortunate that his cleavage terminology was based on Kofoid's four-letter system. Possibly he was influenced by the director and/or advisor for his Ph.D. dissertation work. Certainly the Addendum to Bigelow's 1902 paper, by E. L. Mark and W. E. Castle (pp. 136-137), containing the remarks about four-quadrant "symmetry" finding frequent expression in the cleavage of Lepas, is errant nonsense. In actuality, Lepas shows unit spiral cleavage instead of quadrant spiral cleavage, and this involves monets of micromeres instead of duets or quartets (Costello, 1948). This is not idle conjecture. As Bigelow pointed out, the first cleavage of Lepas is clearly spiral, involving rotation of spindle and protoplasmic areas, inclination of the spindle obliquely with respect to the polar axis, division into micromere and macromere (with the yolk contained in the latter) and subsequent shifting of blastomeres. If one should wish to consider the cleavage of Lepas a "quadrant" form of spiral cleavage, there is but one "quadrant," which, by homology, should be referred to as the "D" (Fig. 6). But of far more significance than finding parallels, as such, between the cleavage patterns of cirripedes, and those of annelids and molluscs, are the differences, which demonstrate that the "purpose" inherent in the cleavage of Lepas is the formation of a bilaterally symmetrical nauplius larva, and not 285 velopmental mechanics of evolution, and the role played by adaptation. "Purpose" is here used in the context employed by Lillie. The views of Wilson (1895, 1898), Conklin (1898), Child (1900) and others have been carefully evaluated also. We shall return to this point below. Much later, Anderson (1969) studied the development of a number of cirripedes from Australia, and while his observations confirm the ones made by Bigelow (1902) on Lepas and by Dels man (1917) on Balanus, he used a different terminology and came to far different conclusions. Anderson (p. 221) states: "The cleavage pattern in cirripede crustaceans hatching as planktotrophic nauplii is indisputably . . . a modified total, spiral cleavage. The first two cleavage divisions segregate four cells as anterior, posterior, left and right quadrant cells, of which the former, B and D, retain transverse contact ventrally while the latter, A and C, retain median contact dorsally in the typical spiral cleavage manner. . . . It is not surprising, therefore, that the spiral cleavage terminology of Wilson (1892) can be applied to the cirripede cleavage sequence with very little modification. The earlier accounts of cell lineage in cirripedes by Bigelow (1902) and Delsman (1917) . . . established most of the cleavage sequence described in the present paper, but both authors obscured the significance of their results by adopting a terminological system which rendered comparison with spiral cleavage almost impossible. .. . The modifications of spiral cleavage displayed in the Cirripedia can now be interpreted as functional changes facilitating cleavage and gastrulation in a small but densely yolky of a trochophore or veliger. The similarities are, perhaps, ancestral reminiscences, while the differences demonstrate the de- egg-" In reality, the obfuscation is equally complete whether one applies Wilson's terminology, as Anderson has done, or retains the terminology of Bigelow and of mesoblasts (ms'bl) divided (bl'po = blastopore); same of later embryo, with mesoblast band (ms'bl) extending anteriorly along dorsal side at right (en'bl = entoblast); profile of 3-metamere stage, with metameres separated by transverse furrows (I, 2,), seen from left side. Fourth row, left to right: Later stage viewed from left side with third furrow (3) superficially dividing mandibular metamere; still later, from left side, with furrow 4 sub-dividing first antennary metamere; dorsal view, same stage, showing longitudinal and transverse furrows which, growing ventrally, fold off the appendages (at1, at2 = antennary rudiments, md = mandibular rudiment); napulius, after developing paired appendages (at1, at2, md) and rudiment of labrum (lbr), seen from left side with ventral surface at top. 286 DONALD P. COSTELLO AND CATHERINE HENLEY Zygote FIG. 6. Table of the cell lineage oiLepas; a translation of the terminologies of Bigelow (1902) (uppermost of each of the three designations), Costello and Henley (this paper) (middle designation) and Anderson (1969) (lowermost designation), for the blastomeres of the 2- and 4-cell stages, and for parts of the 8-, 16- and 32-cell stages. Derivation of two of the secondary mesoblast cells of the 62-cell stage is indicated for Bigelow's scheme. In parentheses are Bigelow's alternative designations for the three single micromeres, the single yolk-bearing cells of the several successive generations, and for the primary and secondary mesoblast. Delsman (Kofoid's scheme) since in either case a four-quadrant terminology is not applicable to a one-"quadrant" system. Bigelow, at least, realized the lack of pertinence of the terminology he was employing, while Anderson clearly does not. Anderson (1969, 1973) describes the presumptive mesoderm of cirripedes as arising from only three "stem-cells" (his 3B, 3A, and 3C) of the 33-cell stage, and the yolk-cell (his 4D) is the presumptive midgut, with all remaining superficial cells the presumptive ectoderm. The mesoderm later forms the musculature of the three paired appendages and leaves a "residual posterior mass" which gives rise to all postnaupliar mesoderm, mainly through the activities of teloblasts (p. 231, 1969). Bigelow, for Lepas, had described at the 62-cell stage as "secondary mesoblast" four cells of the upper three "stories" (his a75, b 75 ,b 77 ,c 7 - 5 )and,as primary mesoblast, the d 52 cell. Anderson (1969, p. 224) is incorrect in stating that Bigelow (and Delsman) "placed much greater emphasis on 4d, posterior to the presumptive midgut, as the main source of mesoderm in cirripede embryos." Bigelow's "primary mesoblast" was, SPIRALIAN DEVELOPMENT: A PERSPECTIVE as noted above, his d 52 cell which, by our translation, would correspond to 2d in Anderson's scheme of nomenclature, not to his 4d. It becomes 4d only in single-"quadrant," monet-type spiral cleavage (our nomenclature, Fig. 6). Anderson's 4d would not appear on the scene until two cell generations later. In the bilaterally symmetrical cirripede nauplius, the paired appendages are developed, beginning anteriorly, at what corresponds to three successive levels or "stories" of the embryo. Since it is essential that both larval mesoderm and ectoderm be provided for each pair of appendages, which are in serial order, it seems logical to expect that these materials would originate from derivatives of the single micromeres of the three generations (i.e., Id, 2d and 3d), located in these positions. There is less logic to deriving the three pairs of appendages from the anterior, right and left quadrant cells (B, C and A derivatives, respectively) of Anderson's terminological scheme. In other words, a terminology fitting the situation should be employed. A crustacean embryo has anteroposterior polarity, and ectodermal and mesectodermal cells covering a single internal endodermal cell, as Bigelow showed. There is not a great architectural gulf between such an embryo and one developing from the centrolecithal egg (of a higher arthropod), with its superficial covering (derived through cortical nuclear divisions) enclosing a central undivided yolk mass. Different methods of arriving at cellulation and diverse arrangements for the manipulation of yolk are presumably part of the adaptation-variation saga. And when one considers the enormous and varied metamorphic changes that occur between arthropod larval and adult stages in different forms, one might expect correspondingly great variations in the early developmental stages within this diverse group. 287 Cleavage through the 4-cell stage is definitely spiral, but the following and later cleavages appear to have lost this characteristic. At the 4-cell stage there is a single large yolk-bearing cell from which all the endoderm is later derived (after six more generations of small cells are given off). Jennings used a four-quadrant nomenclature (that devised by Kofoid, 1894, for Limax), which, of course, would not be applicable in this situation, and describes this single large cell (from his "quadrant D") as "passing within the egg" (p. 54). The ectoderm, derived from the entire "A, B and C quadrants" plus some small cells (23 in number by the eighth generation) from the "D quadrant" covers the endodermal cell. The origin of the mesoblast in rotifers remained uncertain. So here, again, is a case of spiral cleavage seemingly referable to a single "quadrant" but much less clear than for Lepas. There is at least one other group, in addition to the cirripedes and rotifers, in which all the endoderm is derived from a single cell of the 4-cell stage. This is the Gastrotricha, as described for Lepidodermella by Sacks (1955). The cleavage of this form bears many resemblances to that of Lepas. We do not agree with Ivanova-Kasas (1959) that this is a duet form of spiral cleavage. Marked inclination of the spindles in spiral cleavage by quartets begins at the third cleavage; by duets at the second cleavage; and by monets at the first cleavage, as in Lepas and Lepidodermella. Now it is clear that spiral cleavage by quartets leads to the development of a trochophore (or corresponding type) larva. The ideal trochophore, such as that ofHydroides (Fig. 7), has the appearance of a spinning top, with apical tuft and incorporated bilateral features. It is the ciliated prototroch that characterizes the trochophore. In Nereis (Fig. 8) a somewhat simpler trochophore is produced, but with the same general characteristics. During the metamorphosis of the Nereis trochophore larva into the juvenile, the Nectochaeta larva Rotifer (Fig. 9) has accentuated bilateral features, A cleavage pattern somewhat similar to which are characteristic of the adult. This that of Lepas is found in the rotifer brings us again to Lillie and his theory of Asplanchna, as studied by Jennings (1896). adaptation in cleavage. 288 DONALD P. COSTELLO AND CATHERINE HENLEY F. R. LILLIE AND THE THEORY OF ADAPTATION IN CLEAVAGE: A NEW APPLICATION E. B. Wilson (1892, p. 455) once remarked that "The fundamental forms of cleavage are primarily due to mechanical conditions, but are only significant morphologically insofar as they have been secondarily remodeled by processes of precocious segregation." F. R. Lillie agreed (1895; p. 38) and added that "It is parallel precocious segregation in different cases that conditions cell homologies." These observations led him to enunciate his theory of adaptation in cleavage, which stated (p. 39): "These peculiarities of cleavage are all due to the precocious segregation of organs or tissues in separate blastomeres. The order and character of the segregation again are ruled by the needs of the embryo. .. . The peculiarities of the cleavage of Unio are but a reflection of the structure of the glochidium, the organization of which controls and moulds the nascent material." To many embryologists familiar with invertebrate materials, Lillie's principle of adaptation in cleavage is a self-evident truth, yet there are those who consider it to be retroactive reasoning. But from a neoDarwinian viewpoint, with infinite possible variation, selectivity pressure, and relative survival values, it seems clear that production of highly adapted and easily disseminated larval forms could readily contribute to main evolutionary trends. 2 Garstang (1922, p. 82) emphasized, "The real Phylogeny of Metazoa has never been a direct succession of adult forms but a succession of ontogenetic life cycles. Ontogeny does not recapitulate phylogeny, it creates it." And, as has been pointed out (Costello, 1961ft, 1970), only in species with direct development could ontogeny be closely parallel to the phylogeny of adult form. If a biological architect indulging in a flight of imagination envisioned the trochophore (Figs. 7 and 8) as one of the near-ideal ways to disseminate a species, what better means to create this little spinning top could have been devised than on a potter's wheel of cleavage with spiral flourishes? And if, after becoming widely distributed, the trochophores were destined to settle down and become bilaterally symmetrical organisms, would it not be essential that provision for this transition be built in? One may bring this too-simple analogy closer to reality by pointing out that the cleavages are not truly spiral, but oblique, and that the "wheel" merely "rocks" back and forth, alternately dexiotropic and laeotropic. Furthermore, this regular alternation could easily lead to bilaterality in later cleavages by stopping at the halfway point, like a pendulum coming to rest. Lillie (1895, footnote, p. 38) excepts from the principle of adaptation the oblique character of spiral cleavage and its general form. But for what reasons do cleavage patterns vary unless it is to adapt them to the needs of the embryo} Radial cleavage can lead directly to the radially symmetrical ciliated blastula of the echinoderm embryo; spiral cleavage to the trochophore or veliger; and bilateral cleavage to various tadpole-like or other bilaterally symmetrical larvae. If development is telescoped, larval stages are omit2 FIG. 7. Six-day trochophore of Hydroides (after Hatschek, 1886). An essentially similar conclusion was reached by the students of cell lineage of the 1890s (see especially Conklin, 1897, pp. 198-201). But the great significance of adaptation in cleavage appears to have been largely ignored by the more recent generations of embryologists. SPIRALIAN DEVELOPMENT: A PERSPECTIVE 289 turnings and discardings, to achieve the adult stage, so long as each adaptation along the way has had survival value for the species.This appears to be the case for -od fh forms with complex life cycles. Or there can be a minimum of deviousness, with no impedimenta, along the path to the adult, as in the cases of direct development we have discussed. Direct development is itself an adaptation, achieving, as one of its purposes, the conservation of biological materials and energies. st In short, we suggest that somewhere in sd the evolution of development, for whatever reason, some zygote followed a modified mb spiral cleavage pattern which produced, FIG. 8. Twenty-hour trochophore of Nereis (after Wil- with minimum expenditure of material and effort, a successful and viable larva and/or son, 1892). juvenile. For the most efficient production ted, with differentiation to juveniles pre- of a bilaterally symmetrical juvenile, one ceding the growth transition to adults. As an example of such direct development, we can consider that of the squid. Here the cleavage is not spiral as in other molluscs, but bilateral; and there is no larva but rather, direct development to the adult form. Thus it may readily be seen that here the "need" of the embryo is to develop the bilaterally symmetrical condition of the adult as rapidly as possible. There should be no wasting of developmental energies on unnecessary morphogenetic activities. Thinking in terms of the normal transition from spiral to bilateral, about the only way one could homologize cephalopod development with spiral cleavage would be by assuming that when the transition occurs sufficiently precociously, there is no spiral period at all. Bilaterality is, in fact, already present in the unsegmented squid egg. Arnold (1965) reported no evidence of spiral cleavage, even during the first two cleavages.3 So we have come to view the evolution of development as the exploitation of adaptation. Possibilities for variation are nearly infinite. There can be a maximum of twistings and ai. 3 After this account was prepared, a further discussion with Dr. John Arnold revealed that there may, indeed, be some traces of spiral cleavage in the very early development ofLoligo. He will perhaps consider this point in his contribution to the present Symposium. FIG. 9. Nectochaeta larva of Nereis. 290 DONALD P. COSTELLO AND CATHERINE HENLEY such "adaptive adoption" of a cleavage pat- Conklin, E. G. 1898. Cleavage and differentiation. Biol. Lects., MBL, Woods Holl, 1896 and 1897: tern is spiral cleavage by duets, another by Ginn, Boston. monets, a third by purely bilateral cleavage, Costello, D. P. 1937. The early cleavage of Polychoerus which has lost all vestiges of its spiral nacarmelensis. Anat. Rec. 70:108-109. ture. These patterns have persisted and Costello, D. P. 1945. Experimental studies on germinal localization in Nereis. I. The development of isolated survived because they produced a viable blastomeres. J. Exp. Zool. 100:19-66. product, for an existing life-style and Costello, D. P. 1948. Spiral cleavage. Biol. Bull. 95:265 environment. If, anywhere along the line, (see, also, Erratum, Biol. Bull. 95:361). they had taken a wrong turn, so that the Costello, D. P. 1955. Cleavage, blastulation and gatstrulation. /» B. H. Willier, P. A. Weiss and V. resulting larva/juvenile was not successful, Hamburger (eds.), Analysis of development. Saunders, such cleavage patterns would have been Philadelphia. lost. Costello, D. P. 1961a. On the orientation of centrioles Admittedly the viewpoints of emin dividing cells and its significance: A new contribution to spindle mechanics. Biol. Bull. 120:285-312. bryologists are reflections of their personal philosophies. Much depends on whether Costello, D. P. 1961A.Larva, invertebrate. In P. Gray (eA.),Encyclopedia of the biological sciences, edit. 1, pp. they are "splitters" or "lumpers," more con544-549. Reinhold, New York. cerned with differences or with similarities, Costello, D. P. 1970. Larva, invertebrate. In P. Gray and upon how they view broad generaliza(ed.), Encyclopedia of the biological sciences, edit. 2, pp. 481-486. Van Nostrand-Reinhold, New York. tions. But we feel that conjecture can be useful, as well as entertaining, regardless of Delsman, H. C. 1917. Die Embryonalentwicklung von Balanus balanoides Linn. Tijdschr. Ned. Dierk. Ver. whether final solutions to these problems (2) 15: 419-520. are forthcoming. This is our perspective. Fol, H. 1875. Etudes sur le developpement des Mollusques. l er Memoire: Sur le developpement des Pteropodes. Arch. Zool. Exp. Gen. 4:1-214. Gardiner, E. G. 1895. Early development of Polychoerus caudatus Mark. J. Morph. 11:155-176. REFERENCES Garstang, W. 1922. The theory of recapitulation: A critical re-statement of the Biogenetic Law. J. Linn. Anderson, D. T. 1969. On the embryology of the cirSoc. (Zool.) 35:81-101. ripede crustaceans Tetrachta rosea (Krauss), T. pur- Hatschek, B. 1886. Entwicklung der Trochophora von purescens (Wood), Chthamalus antennatus (Darwin) Eupomatus uncinatus, Philippi (Serpula uncinatus). and Chamaesipho columna (Spengler) and some con- Arbeit. Zool. Inst. Wien 6:121-148. siderations of crustacean phylogenetic relation- Heider, K. 1914. Phylogenie der Wirbellosen. In P. ships. Phil. Trans. Roy. Soc. London, B 256:183Hinneberg (ed.), Die Kultur der Gegenwart, part 3, 235. sect. 4, v. 4. Teubner, Berlin. Anderson, D. T. 1973. Embryology and phytogeny in Henley, C. 1974. Platyhelminthes (Turbellaria). In A. annelids and arthropods. Pergamon Press, Oxford. C. GieseandJ. S. Pearse (eds.),Reproduction of marine Apelt, G. 1969. Fortpflanzungsbiologie, Entwickinvertebrates. Academic, New York. lungszyklen und vergleichende Fruentwicklung Ivanova-Kasas, O. M. 1959. [The origin and evolution acoeler Turbellarien. Mar. Biol. 4:267-325. of spiral cleavage].Vestn. Leningrad Univ., no. 9 Arnold, J. M. 1965. Normal embryonic stages of the (Biol.):56-67. (In Russian, with English summary). squid Loligo pealii (Lesuer). Biol. Bull. 128:24-32 Jennings, H. S. 1896. The early development of Ax, P. 1963. Relations and phylogeny of the TurbelAsplanchna herrickii de Guerne. Bull. Mus. Comp. laria./n E. C. Dougherty (ed.),TAelowerMetazoa, pp. Zool. 30:1-117. 191-224. University of California, Berkley. Kato, K. 1940. On the development of some Japanese Bigelow, M. A. 1902. The early development of Lepas. polyclads. Jap. J. Zool. 8:125-143. A study of cell-lineage and germ-layers. Bull. Mus. Kato, K. 1957 (trans. 1968). Platyhelminthes (class Comp. Zool. 40:61-144. Turbellaria). In M. Kume and K. Dan (eds.), InBlochmann, F. 1881. Ueber die Entwicklung der vertebrate embryology. Nolit, Belgrade. Neretinafluviatdis Mull. Zeit. wiss. Zool. 36:125-174. Kofoid, C. A. 1894. On some laws of cleavage in Limax. Bresslau, E. 1909. Die Entwicklung der Acolen. Verh. Proc. Amer. Acad. Arts Sci. 29:180-203. Deutsch. Zool. Ges. 19:314-324. Kofoid, C. A. 1895. On the early development of Bresslau, E. 1933. Turbellaria. In KukenthalUmax. Bull. Mus. Comp. Zool. 27:35-118. Krumbach, Handbuch der Zool. 2 (l):53-320. Lillie.F. R. 1895. The embryology of the Unionidae.J. Child, C. M. 1900. The significance of the spiral type Morph. 10:1-100. of cleavage and its relation to the process of Lillie, F. R. 1899. Adaptation in cleavage. Biol. Lects., differentiation. Biol. Lects., MBL, Woods Holl, MBL, summers of 1897 and 1898. Ginn, Boston. 1899:231-266. Marcus, E. 1958. On the evolution of the animal phyla. Conklin, E. G. 1897. The embryology of Crepidula. J. Quart. Rev. Biol. 33:24-58. Morph. 13:1-226. Remane, A. 1952. Die Grundlagen des Natilrlichen Sys- SPIRALIAN DEVELOPMENT: A PERSPECTIVE terns, der vergleichenden Anatomie und der Phylogenetih. Leipziger Akad. Verlagsges. Remane, A. 1957. Zur Verwandtschaft und Ableitung der niederen Metazoen. Verh. deutsch. Zool. Gesell. 1957 Graz.:179-196. Remane, A. 1963. The systematic position and phylogeny of the pseudocoelomates. In E. C. Dougherty (ed.), The Lower Metazoa, pp. 247-255. Univ. of Calif., Berkeley. Robert, A. 1902. Recherchessurledeveloppementdes Troques. Arch. Zool. Exp. 3e Ser., 10:269-538. Sacks, M. 1955. Observations on the embryology of an aquatic gastrotrich Lepidodermella squamata(Dujar- din, 1841). J. Morph. 96:473-495. Schleip, W. 1929. Die Determination derPrimitiventwck- lung. Akad. Verlags., Leipzig. Treadwell, A. L. 1899. Equal and unequal cleavage in 291 annelids. Biol. Lects., MBL, Woods Holl, 1897 and 1898. Ginn, Boston. Ulrich, W. 1951. Vorschlage zu einer Revision der Grossenteilung des Tierreichses. Verh. deutsch. zool. Gesell., Marburg:244-271. Zool. Anz. Suppl. 15). Whitman, C. O. 1878. The embryology of Clepsine. Quart. J. Micr. Sci. n.s. 18:215-315. Wilson E. B. 1892. The cell-lineage of Nereis.]. Morph. 6:361-480. Wilson E. B. 1895. The embryological criterion of homology. Biol. Lects., MBL, Woods Holl, 1894. Ginn. Boston. Wilson, E. B. 1898. Considerations on cell-lineage and ancestral reminiscence. Ann. N. Y. Acad. Sci. 11:127.