One of the most important features of the segmentation cavity in the Elasmobranchii which I have studied, is the fact that throughout its whole existence its roof is formed of _lower layer cells_. There is not the smallest question that the segmentation cavity of these fishes is the homologue of that of Amphioxus, Batrachians, etc., yet in the case of all of these animals, the roof of the segmentation cavity is formed of epiblast only. How comes it then to be formed of lower layer cells in Elasmobranchii?
To this question an answer was attempted in my paper, "Upon the Early Stages of the Development of Vertebrates[133]." It was there pointed out, that as the food material in the ovum increases, the bulk of the lower layer cells necessarily also increases; since these, as far as the blastoderm is concerned, are the chief recipients of food material. This causes the lower layer cells to encroach upon the segmentation cavity, and to close it in not only on the sides, but also above; from the same cause it results that the lower layer cells assume, from the first, a position around the spot where the future alimentary cavity will be formed, and that this cavity becomes formed by a simple split in the midst of the lower layer cells, and not by an involution.
Footnote 133: _Quart. Journ. of Microscop. Science_, July, 1875. [This Edition, No. VI.]
All the most recent observations[134] on Osseous Fishes tend to shew that in them, the roof of the segmentation cavity is formed alone of epiblast; but on account of the great difficulty which is experienced in distinguishing the layers in the blastoderms of these animals, I still hesitate to accept as conclusive the testimony on this point.
Footnote 134: Oellacher, _Zeit. f. Wiss. Zoologie_, Bd. XXIII. Götte, _Archiv f. Mikr. Anat._ Vol. IX. Haeckel, _loc. cit._
In the formation a second time of a cellular floor for the segmentation cavity in the third stage, the Elasmobranch embryo seems to resemble that of the Osseous Fish[135]. Upon this feature great stress is laid both by Dr Götte[136] and Prof. Haeckel[137]: but I am unable to agree with the interpretation of it offered by them. Both Dr Götte and Prof. Haeckel regard the formation of this floor as part of an involution to which the lower layer cells owe their origin, and consider the involution an equivalent to the alimentary involution of Batrachians, Amphioxus, &c. To this question I hope to return, but it may be pointed out that my observations prove that this view can only be true in a very modified sense; since the invagination by which hypoblast and alimentary canal are formed in Amphioxus is represented in Elasmobranchii by a structure quite separate from the ingrowth of cells to form the floor of the segmentation cavity.