In this case we would be compelled to suppose that the mass of lower layer cells which forms the embryonic swelling is used as food for the growth of the involuted epiblast, or else employed solely in the growth over the yolk of the non-embryonic portion of the blastoderm; but the latter possibility does not seem compatible with my sections.
I do not believe that it is possible, from the examination of sections alone, to decide which of these two views (viz. whether the epiblast is involuted, or whether it becomes merely continuous with the lower layer cells) is the true one. The question must be decided from other considerations.
The following ones have induced me to take the view that there is no involution, but that the mesoblast and hypoblast are formed from the lower layer cells.
(1) That it would be rather surprising to find the mass of lower layer cells which forms the “embryo swelling” playing no part in the formation of embryo.
(2) That the view that it is the lower layer cells from which the hypoblast and mesoblast are derived agrees with the mode of formation of these two layers in the Bird, and also in the Frog; since although, in the latter animal, there is an involution, this is not of the epiblast, but of the larger cells of the lower pole of the yolk, which in part correspond with what I have called the lower layer cells in the Dog-fish.
If the view be accepted that it is from the lower layer cells that the hypoblast and mesoblast are formed, it becomes necessary to explain what the continuity of the hypoblast with the epiblast means.
The explanation of this is, I believe, the keystone to the whole position. The vertebrates may be divided as to their early development into two classes, viz. those with holoblastic ova, in which the digestive canal is formed by an involution with the presence of an “anus of Rusconi.”
This class includes “Amphioxus,” the “Lamprey,” the “Sturgeon,” and “Batrachians.”
The second class are those with meroblastic ova and no anus of Rusconi, and with an alimentary canal formed by the infolding of the sheet of hypoblast, the digestive canal remaining in communication with the food-yolk for the greater part of embryonic life by an umbilical canal.
This class includes the “Elasmobranchii,” “Osseous fish,” “Reptiles,” and “Aves.”