Download EXPERIMENTS ON EMBRYONIC INDUCTION

VOL. XI, No. 3
JULY, 1934
EXPERIMENTS ON EMBRYONIC INDUCTION1
PART I. THE COMPETENCE OF THE EXTRA-EMBRYONIC
ECTODERM IN THE CHICK
PART II. EXPERIMENTS ON COAGULATED ORGANISERS
IN THE CHICK
PART III. A NOTE ON INDUCTIONS BY CHICK PRIMITIVE
STREAK TRANSPLANTED TO THE RABBIT EMBRYO
BY C. H. WADDINGTON.
(Strangeways Research Laboratory and Sub-Department of
Experimental Zoology, Cambridge.)
(Received $th September, 1933.)
(With Two Plates and Two Text-figures.)
I. THE COMPETENCE OF THE EXTRA-EMBRYONIC ECTODERM
IN THE CHICK.
INTRODUCTION.
IT has been shown (Waddington, 1933 a) that the tissue movements which lead to
the formation of the primitive streak in birds are determined by the gut endoderm.
And further it is known that in the primitive streak stage the ectoderm of the area
pellucida can react, by the formation of an induced neural plate, to the presence of a
piece of primitive streak. The area pellucida ectoderm is in fact" competent" to form
neural plate (Waddington, 1932). The question arises as to how this competence is
determined. The question is of considerable importance, since all recent investigations have tended to show that in the process of embryonic induction the inducing
factors merely release the developmental processes present in the reacting tissue,
allowing them to proceed in one among the various ways open to them.
The first hypothesis which suggests itself is that the competence is determined
by the underlying gut endoderm at the same time as the determination of the primitive streak. If this hypothesis is true, the competence should not be possessed by
the ectoderm of the area opaca, since this is not underlain by the gut endoderm but
only by the germ wall, a tissue which differs from the gut endoderm both in origin
and in histological type.
A series of transplantations of pieces of primitive streak into the area opaca have
therefore been made, using chick embryos. A high proportion of successful inductions have been obtained, some of which are here described. The fact that such
inductions are possible shows that the area opaca ectoderm does possess the com1
This work was done while I was in receipt of a part-time grant from the Medical Research
Council, and an expenses grant from the Royal Society, for which I should like to make grateful
acknowledgment.
lEB-xiiii
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Experiments on Embryonic Induction
213
The graft tissues cannot be certainly identified, but are fused with the overlying
tissues to form a single "induced axis."
33-76. Host.
18 hours, L pr. s.
Donor.
18 hours, LM pr. s.
Graft.
Anterior half of pr. s., no end. dv. aa. in a. op. at right side.
Cultivated. 24 hours.
The graft has moved round so as to lie approximately south-west to north-east.
The anterior end of the graft and induced structures reach to the outer edge of the
area vasculosa, while the posterior end forms a process projecting above the surface
of the blastoderm; this process becomes free from the main part of the blastoderm
at about the boundary between the area pellucida and the area vasculosa opaca.
_. _ — H.emb.
'
area,
pellucida.
'—a.Qpaca,.
I.hf.
Text-fig. 1. 33-3 Diagrammatic sketch of whole specimen. H. emb. host embryonic axis;
I.fgt induced fore-gut; /. hf. induced head-fold.
The sections show a large induced neural plate which in its anterior, north-east,
end is underlain by graft neural plate and probably graft mesoderm, although the
boundaries of the latter cannot be made out with certainty (PI. I, fig. zd). There is
little sign of the formation of a head by the induced plate, although there is a small
fold which might be interpreted as a rudimentary head-fold. At the posterior, southeast, end the induced plate extends along the upper surface of the projecting process,
the interior of which contains a strand of mesoderm which is probably derived from
the graft (PI. I, fig. zb).
33-77. Host.
Donor.
Graft
Cultivated.
18J hours, LM pr. s.
18 hours, LM pr. s. (= donor to 33-76).
Posterior half of pr. s. no end. in a. op. at right anterior.
24 hours.
15-2
214
C. H. WADDINGTON
The graft has formed a mass of blood, lying at the edge of the area vasculosa. The
regular distribution of the host's blood islands is interrupted and the graft can be
seen as a central mass of blood in a small blood-free area which may also be made up
partly of graft tissue. No induction has occurred.
33-84. Host.
18J hours, M pr. 8.
Donor.
18J hours, LM pr. s.
Graft.
Posterior two-thirds of pr. s. in a. op. at left anterior.
Cultivated. 2i£ hours.
The graft and induced structures lie to the left of the host embryo. They are just
outside the edge of the area vasculosa, and are thus completely within the area
vitellina. There is a large induced neural plate, the anterior part of which runs south
to north, while the posterior part is curved round so as to lie nearly west to east
(PI. I, fig. 3a). The anterior part probably represents a head, as there is a small
head-fold. Graft neural tissue is present in the anterior region (PI. I, fig. 36), but
absent in the posterior part of the induction, where the induced plate is underlain
only by mesoderm (somites and probably notochord) (PI. I, fig. 3 c).
33-85. Host.
Donor.
Graft.
i8f hours, LM pr. s.
18$ hours, LM pr. s.
Anterior half of primitive streak, dv. in a. op. at right
anterior, west to east.
Cultivated. 21J hours.
The graft has shifted considerably during development, and in the fixed specimen
it runs nearly south to north. The graft and induced structures lie wholly within the
area vasculosa. The induced plate is flat and unfolded throughout its length (PI. I,
fig. 4). At its most anterior, north, end there is a slight indication of a head-fold.
The plate is underlain by masses of graft mesoderm, which are considerably complicated by the blood sinuses of the host. Among this mesoderm, graft neural tissue,
accompanied by notochord, can be distinguished at the anterior end.
33-86. Host.
Donor.
Graft.
19 hours, L pr. s.
19 hours, L pr. s.
Anterior third of pr. s. in a. op. at right posterior, west to
east.
Cultivated. 21 hours.
The host embryonic axis entirely failed to develop, but the area vasculosa was
fairly normal. The graft has given rise to a large neural plate accompanied by notochord and somites. Throughout most of its length, this has called forth from the
overlying ectoderm no reaction except a very slight thickening, but the ectoderm
above the most anterior, east, part of the graft has formed a small thick neural plate
(PI. I, fig. 5). This lies in the area vitellina, just outside the area vasculosa.
33-116. Host.
18 hours, LM pr. s.
Donor.
18 hours, SM pr. s.
Graft.
Whole pr. s. dv. ap. no end. in a. op. at left anterior.
Cultivated. 28 hours.
Experiments on Embryonic Induction
215
The host blastoderm was inadvertently cut longitudinally along the primitive
streak during the operation, and the two sides of the embryonic axis are widely
separated.
The graft lies just at the periphery of the area vasculosa. It forms a mass of meaoderm, which is chiefly undeveloped blood. There is a slight projection developed
in connection with the posterior part of the graft. No induction has occurred.
33-117. Host.
18\ hours, SM pr. s.
Donor.
18J hours, SM pr. s.
Graft.
Anterior half of pr. s. dv. ap. no end. in a. op. at right.
Cultivated. 32 hours.
A large induced neural plate was visible in the fixed and stained specimen. Part
of it lies within the area pellucida but the posterior end stretches well into the area
H.emb.
''I.emb.
•-edge of
area, pellucida.
Text-fig. 2. 33-117. Diagrammatic sketch of whole specimen. A section along the line marked
6 is reproduced in PI. II, fig. 6. pro. process; H. emb. host embryonic axis; /. emb. induced embryonic axis.
opaca vasculosa. The induced plate has a sinuous course which can be seen from the
figure (Text-fig. 2). The most remarkable feature is found at the anterior end, which
lies towards the south. In this region, the induced plate is connected with the host
plate by a sort of "bridge," a short piece of neural plate running approximately
east to west. The origin of this bridge is obscure; probably it is derived more or less
equally from host and induced tissues, but some sections suggest that graft neural
tissue also has united secondarily with it.
The most anterior part of the induced plate does not form a head, as is shown
by the absence of fore-gut or head-fold; it probably has the regional character of
that part of the host plate with which it is connected. The posterior end of the induced plate lies on the upper surface of a process which projects from the surface
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C. H. WADDINGTON
of the blastoderm. Owing to a bend in the course of the induced plate, it is cut twice
in the section from this region shown in PI. II, fig. 6.
33-121. Host.
2o| hours, LM pr. s.
Donor.
? hours, early head-fold.
Graft.
(a) Anterior part of neural plate, with underlying mesoderm and endoderm, dv. in a. op. at left middle.
(b) Hensen's node dv. ap. at right middle.
Cultivated. 25J hours.
The graft (b) has performed an induction, but as it lies within the area pellucida
it will not be described here.
Graft (a) has formed a large tubular mass of neural tissue, which is accompanied by notochord at its south end; small tubules of graft endoderm can also be
seen. Overlying the graft throughout its length, except at its extreme north end, is
a rather small induced neural plate (PI. II, fig. 7). There is no induced notochord,
nor any other induced structure.
DISCUSSION.
No discussion of the main results described in this communication seems to
be called for. A clear-cut problem was proposed, namely, whether the ectoderm
of the area opaca is competent to form neural plate; and a clear-cut affirmative
answer has been given. It may therefore be asserted that the competence to form
neural plate is not conferred on the ectoderm of the chick blastoderm by the gut
endoderm. The origin of the competence must for the present remain unknown.
The notion of competence is allied to that of potency. Both are defined with
reference to future possibilities. Woodger (1929) has pointed out the danger of
regarding potencies as real entities: they are states characterising particular parts
of the embryo. The notion of competence has from the beginning been defined
adjectivally in this way: a given piece of tissue is competent to form a tissue or organ
X during that period of time within which it becomes determined either to form X
or not to form X (Waddington, 1932). Translating this into material terms of a
material model, it might be supposed that during the period of competence to form
X, there is present in the competent tissue a certain constellation of chemical substances or processes which are in unstable equilibrium, so that the addition of small
quantities of other substances (the organiser substances) can cause the whole
equilibrium to break down, and the system to alter progressively in a certain direction towards some new equilibrium. On the basis of this speculative model, competence would be regarded as a state of unstable equilibrium. It might be suggested
that it is hopeless to search for its origin in factors external to the particular piece
of tissue in question, but that it is more probably produced by the interaction of
factors present in the tissue itself; and these factors may be particularly closely
associated with the genes.
There are one or two remarks which it may be profitable to make concerning
various points of interest illustrated in the specimens described.
Experiments on Embryonic Induction
217
Specimen 33-3 is the first example of an induction of a head in the posterior
region of the host blastoderm. It provides a remarkably good demonstration of the
" dominance " of the anterior part of the individuation field over the posterior part.
This dominance has previously been proved for the Amphibia (Spemann, 1931a)
and assumed for the birds (Waddington & Schmidt, 1933).
The formation of projecting processes in connection with induced neural plates,
as for example in 33-76, 33-117, and other previously described specimens, would
probably repay a special investigation. My impression is that such processes are
only produced when the graft induces structures which correspond to the anterior
and middle parts of the primitive streak but not those corresponding to the posterior
parts. The structures induced by such anterior grafts would have the normal
tendency for posterior elongation in the late primitive streak stage of development,
but the material posterior to them would not have the concomitant tendency to move
out of the way, so that the elongating induced structures would be forced to become
dissociated from the main part of the blastoderm, forming a process.
In the specimens 33-77, 33-116, no induction has taken place, and the grafts,
which consisted of middle or posterior pieces of the primitive streak, have differentiated only into blood. This type of differentiation probably also occurs in grafts
into the area pellucida but seems to be more commonly found in grafts into the area
opaca. It might be regarded as an expression of the individuation field of the host,
since blood is a tissue proper to this part of the blastoderm, but it is perhaps more
likely to be due to some more general physiological conditions. Murray (1932) has
observed the formation of blood from the posterior three-quarters of the primitive
streak during cultivation by the cover-glass method,using a liquid medium. Probably
in both cases a " bedeutungsfremde " differentiation has taken place.
Specimen 33-117 gives a remarkable example of the tendency for two regions
of neural plate to become continuous with one another. This tendency is frequently
observed, but the reasons for it are obscure.
No fore-gut has been induced in yolk-sac endoderm or germ wall. This is not
surprising when the histological characteristics of those tissues are taken into consideration. The only induced fore-gut in the specimens described is that of 33-3,
which seems to be formed from gut endoderm derived from the edge of the area
pellucida. In fact, as was remarked in the description, it is doubtful whether this
specimen really shows an induction in the area opaca at all; it is quite probable that
the effect of the graft has been first to cause the host primitive streak to elongate
posteriorly in an abnormal direction, and then to alter the regional character of this
part of the host axis to " head."
SUMMARY.
It is shown that the ectoderm of the area opaca of chick embryos of the primitive
streak stage can react to primitive streak grafts by the formation of an induced neural
plate. The conclusion is drawn that the competence to form neural plate is not
conferred on ectoderm by the gut endoderm, which determines the formation of
the primitive streak.