DEVELOPMENT OF ELASMOBRANCH FISHES. 45 



still remain (PI. in. figs. 2, 3, 4). During the third period of 

 its history, a distinct cellular floor is again formed for it, so 

 that it comes a second time into the same relations with the 

 blastoderm as at its earliest appearance. The floor^ of cells 

 which it receives is in part due to a growth inwards from the 

 periphery of the blastoderm, and in part to the formation of 

 fresh cells from the yolk. Coincidently with the commencing 

 differentiation of hypoblast and mesoblast the segmentation 

 cavity grows smaller and vanishes. 



One of the most important features of the segmentation 

 cavity in the Elasmobranchs which I have studied, is the fact 

 that throughout its whole existence its roof is formed of loiver 

 layer cells. There is not the smallest question that the seg- 

 mentation 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 w^as attempted in my paper, 

 "Upon the Early Stages of the Development of Vertebrates^" 

 It was there pointed out, that as the food material in the ovum 

 increases, the bulk of the lower layer cells necessarily also in- 

 creases ; 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 wdiere 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. 



All the most recent observations" on Osseous Fishes tend 

 to show that in them, the roof of the segmentation cavity is 

 formed alone of epiblast ; but on account of the great difficulty 

 w^hich is experienced in distinguishing the layers in the blasto- 

 derms of these animals, I still hesitate to accept as conclusive 

 the testimony on this point. 



1 Quart. Journ. of Microscop. Science, July, 1875. 



2 OeUacher, Zeit. f. Wiss. Zoologie, Bd. xxiii. Gotte, Archiv f. Milr. 

 Anat. Vol. ix. Haeckel, loc. cit. 



