304 DEVELOPMENT OF ELASMOBRANCH FISHES. 



The cavity anteriorly has a more or less definite form, having 

 lateral walls, as well as a roof and floor (PI. 7, figs, lob and io<r). 

 Posteriorly it is not nearly so definitely enclosed (PI. 7, fig. io<?). 

 The ventral wall of the cavity is formed by yolk. But even in 

 stage B there are beginnings of a cellular ventral wall derived 

 from an ingrowth of cells from the two sides. 



By stage C considerable progress has been made in the 

 formation of the alimentary canal. Posteriorly it is as flattened 

 and indefinite as during stage B (PI. 10, figs. 2b and 2c). But 

 in the anterior part of the embryo the cavity becomes much 

 deeper and narrower, and a floor of cells begins to be formed for 

 it (PI. 10, fig. 2) ; and, finally, in front, it forms a definite space 

 completely closed in on all sides by cells (PI. 10, fig. 2 a}. Two 

 distinct processes are concerned in effecting these changes in the 

 condition of the alimentary cavity. One of these is a process of 

 folding off the embryo from the blastoderm. The other is a 

 simple growth of cells independent of any folding. To the first 

 of these processes the depth and narrowness of the alimentary 

 cavity is due ; the second is concerned in forming its ventral 

 wall. The combination of the two processes produces the peculiar 

 triangular section which characterises the anterior closed end of 

 the alimentary cavity at this stage. The process of the folding 

 off of the embryo from the blastoderm resembles exactly the 

 similar process in the embryo bird. The fold by which the 

 constricting off of the embryo is effected is a perfectly continuous 

 one, but may be conveniently spoken of as composed of a head- 

 fold and two lateral folds. 



Of far greater interest than the nature of these folds is the 

 formation of the ventral wall of the alimentary canal. This, as 

 has been said, is effected by a growth of cells from the two 

 sides to the middle line (PI. 10, fig. 2). The cells for this 

 are however not derived from pre-existing hypoblast cells, but 

 are formed spontaneously around nuclei of the yolk. This fact 

 can be determined in a large number of sections, and is fairly 

 well shewn in PI. 10, fig. 2 na. The cells are formed in the 

 yolk, as has been already mentioned, by a simple aggregation of 

 protoplasm around pre-existing nuclei. 



The cells being described are in most cases formed close to 

 the pre-existing hypoblast cells, but often require to undergo a 



