258 DEVELOPMENT OF ELASMOBRANCII FISHES. 



Its earliest appearance is involved in some obscurity, though 

 it probably arises as a simple cavity in the midst of the .lower 

 layer cells (Ph 7, fig. i). In its second phase the floor ceases 

 to be formed of lower layer cells, and the place of these is 

 taken by the yolk, on which however a few scattered cells 

 still remain (PI. 7, 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 lower 

 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 was attempted in my paper, 

 "Upon the Early Stages of the Development of Vertebrates 1 ." 

 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 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. 



All the most recent observations 2 on Osseous Fishes tend 



1 Quart. Journ. of Microscop. Science, July, 1875. [This Edition, No. VI.] 

 - Oellacher, Zeit. f. Wiss. Zoologie, Bd. xxm. Gotte, Archiv /. Mikr. Anat. 

 Vol. IX. Haeckel, loc. cit. 



