Download diversity of individual forms.* All these parts arise in exactly the

100
ON THE DEVELOPMENT OF ASCAKIS NIGB.OVENOSA.
diversity of individual forms.* All these parts arise in
exactly the same way, by budding, and when incompletely
developed are indistinguishable from each other. The germcapsules arise from animal-cells, whose tentacular apparatus
and alimentary organs are aborted. The embryonic development of the Bryozoa, is, speaking generally, very diverse,
since it may be effected not only by fertilized ova and statoblasts, but occasionally also as in Lepralia, by gem mules
springing singly from the inner wall of the animal-cells, or of
the ovicells. The formation of the rest of the contents of the
cell (the digestive and respiratory apparatus, the sexual
organs, and the statoblasts) also takes place, according to
M. Smitt, by gemmation, so that he is disposed to assign to
the Bryozoa (Polyzoa) a double polymorphism, an external
and an internal, the former having reference to the cells and
the latter to the viscera, which also may be more or less individualised, as has already been pointed out by Allman."
In the paper of which the above is a summary, the author'
investigations were conducted in Crisia aculeata, Alcyonidium
gelatinoswm, A. parasiticum, Flustrella hispida, (Etea truncata,
Eucratea chelata, Scrupocellaria scruposa, Canda reptans,
Flustra truncata, Fl. membranacea, and several species of
Membranipora and Lepralia.
In the memoir whose title heads this notice, M. Smitt
describes a new species of CEtea in the following terms :—
CE. argillacea, n. sp.
CE. elongata, recta, punctata, basi constricta.
Hab. In mari Bahusiensi, nullo alio loco/^ut videtur, adhuc reperta; per Modiolam oculina, affixam serpens inventa
est. (Mus. Holm. Loven).
Species (Eteee ligulatse (Busk), maxime affinis, a qu& tamen
facile basi sua constrieta dignoscitur. Longitudo testae
erectse circ. 1'5 mm., cujus dimidiam partem superiorem,
tenet apertra testae obliqua.
On the DEVELOPMENT of ASCARIS NIGROVENOSA.
IN our last number we gave a notice of some late researches
on the development of Ascaris nigrovenosarf by Herr E. C.
Mecznikow, who at the same time claimed to be the original
* In this class, all the six forms of cells (so termed) are certaiuly, not
unfreqiiently, co-existent in the same polyzoary.—G. B.
t 'Quart. Journ. Mic. Sci./ Jan., 1866, p. 25.
ON THE DEVELOPMENT OF ASCAR1S NIGROVENOSA.
101
discoverer of the curious circumstance that that nematode
is capable of sexual reproduction, both in the parasitic and
its free state, in which latter condition it resembles the genus
Rhabditis.
This claim on the part of Herr Mecznikow has, as might
be expected, called forth a reclamation from Professor
Leuckart, who was thus, as it were, inferentially charged
with having appropriated to himself in his previous communication some of the results of his pupil's independent researches. In a recent paper* Professor Leuckart indignantly
repudiates this charge, and, although he allows that the particular fact with respect to A. nigrovenosa was first noticed
by Herr Mecznikow, this only happened in the course of an
investigation into the life-history of the Nematoda which was
carried on by that observer under his own immediate auspices
and directions, and in his own laboratory and with the aid of
his own materials. A reply to this reclamation of Professor
Leuckart has been published in a separate formf by Herr
Mecznikow, and, upon consideration of the whole case, it
appears to us that, although Professor Leuckart, might, perhaps, have been more liberal in his acknowledgment of the
assistance afforded him in his researches on the subject of the
Nematoda by his quondam pupil, still that the latter has
claimed rather more originality than he is entitled to, seeing
that, although he actually observed the fact of the dimorphic
sexuality of A. nigrovenosa, he was led to this observation
during an investigation directed in a course pointed out by his
distinguished teacher. But leaving this very unpleasant and
unprofitable subject, we would draw attention to some of Professor Leuckart's observations upon the other contents of
Herr Mecznikow's highly interesting communication.
With respect to the " cuticular lip " mentioned by Mecznikow in the embryo of A. nigrovenosa, Professor Leuckart
remarks that it is not a continuous structure or border around
the oral orifice, but composed of three distinct papillae, as in
all other nematode embryos hitherto observed by him. The
rudimentary sexual organ, whose considerable size and high
degree of development forms so characteristic a feature of
this Ascaris-embryo, is an elongated body about 0-08 mm.
in length, and 0"012 mm. broad, and containing, not a protoplasm filled with nuclei, but distinctly isolated, though membraneless cells, 0*007 mm. to 0008 mm. in diameter, and
* ' Archiv. f. Anat.,' No. 6, 1865, p. 641.
f 'Entgegung auf die Erwiderung des Herru.'
Gottingen, 1866.
Prof. Leuckart, &c.
102
ON THE DEVELOPMENT OF ASCARIS NIGROVENOSA.
furnished with a vesicular nucleus O0048 mm. in size. In
the immature embryo these cells exactly resemble those of
the intestinal epithelium, their only further change consisting
in their eventually becoming more transparent.
With respect to the sexually mature Rhabditis-form, he
observes that the pharyngeal walls are by no means muscular
throughout their whole extent, as described by Herr Mecznikow. Radial muscular fibres can only be noticed in two situations in them, viz., in the hinder enlargement, where they serve
for the movement of the three chitinous teeth; and more in
front, almost in the middle of the more cylindrical cesophageal
tube, at which point the chitinous covering is also developed
into a sort of armature. The caudal papillae also in the male
are not hair-like, but tolerably thick and conical in form.
The larger-sized female, which in summer usually exceeds
1 mm. in length, has quite as distinct a nervous ring as the
male, although this organ is by no means so distinctly defined in A. nigrovenosa as in many of the nematodes. The
female organs are imperfectly described by Herr Mecznikow.
They do not consist, as asserted by him, of a membraneless
string of ova, but of two elongated sacculi, which stretch forwards and backwards from the genital opening; and at the
time of copulation, besides the vagina, two other divisions of
the sexual tube may be recognised, viz., a uterus and an
ovary. The former represents a tolerably thick, short canal,
of narrow calibre, and apparently having cellular walls, whilst
the ovary is formed by a very delicate, but nevertheless distinctly demonstrable, structureless membrane; and its interior
is filled with ova.
He further remarks that, although the description given by
Herr Mecznikow of the embryos is in the main quite correct, that observer has overlooked the interesting fact that
these embryos, whilst they are within the emptied body of
their parent, present the Rhabditis-form of pharynx, possessing not only the two characteristic enlargements, but also
furnished with three chitinous teeth, smaller, it is true, than
they are in the preceding generation, but of the same form,
and, like them, moved by distinct muscular fibres. When
liberated from the maternal body these teeth are lost, the
muscular striae disappear, and the pharynx assumes a more
Ascaridan form; the creature at the same time has become
capable of being developed in the lungs of the frog into the
trell-known A. nigrovenosa.
Professor Leuckart then describes experiments with respect
' to the introduction of the liberated Ascaridan embryos into
the lungs of the frog. This experiment, it would seem, is
ON THE DEVELOPMENT OF ASCA.RIS NIGB.OVENOSA.
103
not always successful; bat sufficiently often to show that the
lungs are the true destination of the embryos, which, if
swallowed, invariably perish after a time in the stomach.
Professor Leuckart has carefully traced the development of
the embryos into the perfect A. nigrovenosa in the frog's lung,
and has found that they are all invariably females, so that
there can be no doubt that the production of young in the
parasitic Ascaris is entirely parthenogenetic. It is beyond
dovibt also, he says, that this mode of parthenogenesis is
widely diffused among the nematodes, and cites as a tolerably
certain instance of it the case of Filaria medinensis. With
respect to which species, he remarks, that from Carter's
observations, it would seem probable that Filaria medinensis,
like A. nigrovenosa, exhibits two kinds of generations —
a parasitic and a free, and that thus it would present an
exact analogy with the parasite of the frog's lung.
He is of opinion, however, that this notion is erroneous.
And he is led to think so from the circumstance not only of
the slight degree of development of the embryonal rudimental
reproductive system, but further, from the striking similarity
between the embryos of F. medinensis and those of CucuUanus
elegans. According to all analogy, the embryo of F. medinensis
is equally destined to migrate as is that of CucuUanus,
though whether this migration is confined to the human
subject or not it is impossible to say.
At present, he says, notwithstanding his pretty extensive
experience on the subject of the developmental history of the
Nematoda, that of Ascaris nigrovenosa stands alone.
On this account it is the more interesting to record the
same phenomena in other groups of the lower animals,
amongst which he notices the extraordinary fact discovered
by Hackel, of the production, within the visceral cavity of the
mature Geryonice, by a process of budding, of Medusoids of
quite another organization [Cuninci), which also in their turn
reach sexual maturity. He adverts next to the life-history of
Coccus formerly described by himself.* In this case, as in
A. nigrovenosa, two successive generations of different kinds
are thrown off, both of which become sexually developed,
and both of which exist under different conditions. It is true
that the vital conditions in the Aphis-like winged and the
Coccws-like wingless generations are not so strikingly different as in A. nigrovenosa; but the difference between the
two cases is only one of degree, and as such points distinctly
enough to the analogy which exists between them. It is
remarkable, also, that in Chermes the dimorphism of the suc• ' Archiv f. Naturgesch,' 1859, p. 20S.
104
ON THE DEVELOPMENT OF ASCARIS N1GROVENOSA.
sessive sexual generations is combined with the phenomena
of parthenogenesis, which in this case is even exhibited in
both generations.
For this mode of development, with the intervention of two
sexual generations, which, on account of the sexual perfection
of the intermediate generation, does not come under the same
category with the usual form of " Alternation of Generation,"
Professor Leuckart proposes henceforth to employ the term—
HETEROGONY, a word which, it is true, has been otherwise
applied, but which, in its present sense, implies pretty closely
what it was intended to express by its first employer, Johannes
Miiller. Whether Hackel's ALLJEOGONESIS should be included
under Heterogony is at present doubtful, and can only be
decided when -we learn the fate of the offspring derived
from the fertilised ova of the two generations. But, however
this may turn out, we have clearly in this case, as iu Chermes
and Ascaris nigrovenosa,a.t any rate an instance of two different
sexually developed generations which form links in the development of one and the same species.
Hitherto, he concludes by observing, we have been accustomed to regard sexual reproduction, not only as the end and
aim of animal life, but also as the criterion of specific individuality. But neither of these assumptions is any longer
admissible.
" Nature follows its course, and what at one time appears
as an exception becomes a law."