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GROWTH IN LENGTH OF THE FROG EMBRYO.
On the Growth in Length of the Frog Embryo.
Richard Assheton, M.A.
With Plates 23 and 24.
In some previous papers on the development of the rabbit I have attempted to show that there are two main centres of growth, each in itself tending to produce a radially symmetrical form; but since these two centres of growth are situated eccentrically to each other the resulting embryo is cylindrical, and subsequently bilaterally symmetrical.
I endeavoured to show that no concrescence occurred in the rabbit, and that no theory of concrescence was necessary to account for the facts.
I wish now to indicate the manner in which two centres of growth can also bring about the corresponding results in the frog embryo, without any concrescence of the dorsal lips of the blastopore.
In a paper in this Journal Dr. Robinson and I discussed the question of the formation of the archenteron in the frog, and came to the same conclusion as Moquin-Tandon (in Anura) and Houssay (in Axolotl), that the archenteric cavity was due to a splitting amongst the cells in situ, and not to an invagination or overgrowth of surface cells.
Recently Jordan, and Umé Tsuda and Morgan have made very interesting communications upon the subject.
The former author, after an able summing up of the evidence on both sides, concludes (p.331), “The evidence thus far adduced
for invagination is, to say the least, inconclusive;" and on the other hand (p. 332), “ There is not a shred of evidence to show that the large cells at first surrounding the mouth of the blastopore are not subsequently pushed in by the ingrowth of ectoblast cells. No positive evidence whatever exists to prove either the impossibility of invagination or the likelihood of no invagination. I find it difficult to gather the reasons that have influenced Houssay and Robinson and Assheton to adopt the view that invagination does not occur."
Jordan then describes (pp. 333–4) “ocular evidence that the small cells around the lips of the blastopore are actually infolded."
Morgan, after a description of interesting experiments after the method of Roux upon the living egg, says, “ The statement of Robinson and Assheton that no portion of the archenteron in the anura is formed by invagination is certainly incorrect, as I hope to show in a later paper.”
Of course this latter statement must depend upon the exact meaning to be attached to the word archenteron.
My own conception of the term archenteron is that cavity which in the embryo is supposed to represent the digestive cavity of a hypothetical ancestral "gastrula,” no matter how this cavity was brought about.
If, however, by archenteron is meant any subsequent prolongation of this cavity, such as would represent a post-gastrula condition ancestrally, then certainly such a statement was inaccurate.
When Dr. Robinson and I made the statement referred to, we regarded as archenteron part of the cavity which I now consider to represent a post-gastrula condition. In other words, I agree with Morgan and others to a certain extent as regards the growth over of the dorsal lip of the blastopore, and consider only the most anterior part of the gut cavity of the frog's embryo at the time of the closure of the blastopore as being formed by a splitting, and as representing the true archenteron.
Accordingly, in my opinion, the sentence (referred to above) by itself accurately describes the facts, but in the context in
which it stood I admit that I now think it inaccurate. My reasons for so thinking I will now proceed to give.
Again, Morgan and Umé Tsuda say that we "apparently at the outset have orientated the embryo wrongly, for they state the segmentation cavity has a roof which ultimately becomes the anterior wall of the gastrula ; for the anus which marks the posterior end of the embryo appears at the opposite side of the ovum,—that is, on the floor of the segmentation cavity.”
I cannot understand their objection to this paragraph.
Figs. 7 to 11 on Pl. 24 are all placed with what we conceive to be the dorsal surface (D.) directed towards the top of the plate.
As regards the ocular evidence of an invagination spoken of by Jordan, it is a pity that more details are not given of the observations.
Is it possible to trace a cell, or a spot on the surface some distance from the lip of the blastopore, to gradually approach and fold over the edge and so disappear, or do only cells actually on the edge seem to be affected by the process ?
What is the cause of the invagination ? I can quite well imagine that individual cells at the edge may, by multiplication of their neighbours or themselves, be pushed over the edge, as also might cells on the inner edge appear to be pushed outwards, if we could see that edge. The splitting theory still seems to me to be the more probable for the commencement of the archenteric cavity and its extension forwards.
But, as I shall point out a little further on, there is un. doubtedly an apparent overgrowth, and I think certainly an actual overgrowth of the lower pole cells by the dorsal lip of the blastopore, together with the lateral and ventral lips, as they are formed at a later period. This process, however, should not, I think, be compared with the process of gastrulation so called, or formation of primitive archenteron, but should be considered to be intimately connected with the growth in length of the embryo. In other words, to follow the same line of argument that I have used in the description of the rabbit embryo, the
vol. 37, PART 2.-NEW SER.