DEVELOPMENT OF THE ELASMOBRANCH FISHES. 69 



Formation of tJie Layers. 



Leaving these nuclei, I will now pass on to the formation 

 of the layers. 



At the close of segmentation the surface of the blasto- 

 derm is composed of cells of a uniform size, which, however, 

 are too small to be seen by the aid of the simple micro- 

 scope. 



The cells of this uppermost layer are somewhat columnar, 

 and can be distinguished from the remainder of the cells of the 

 blastoderm as a separate layer. This layer forms the epiblast ; 

 and the Dog-fish agree with Birds, Batrachians, and Osseous 

 fish in the very early differentiation of it. 



The remainder of the cells of the blastoderm form a 

 mass, many cells deep, in which it is impossible as yet or 

 till a very considerably later period to distinguish two layers. 

 They may be called the lower layer cells. Some of them 

 near the edge of this mass are still considerably larger than 

 the rest, but they are, as a whole, of a fairly uniform size. 

 Their nuclei are of the same character as the nuclei in the 

 yolk. 



There is one point to be noticed in the shape of the blas- 

 toderm as a whole. It is unsymmetrical, and a much larger 

 number of its cells are found collected at one end than at the 

 other. This absence of symmetry is found in all sections 

 which are cut parallel to the long axis of the egg-capsule. 

 The thicker end is the region where the embryo will subse- 

 quently appear. 



This very early appearance of distinction in the blasto- 

 derm between the end at which the embryo will appear, and 

 the non-embryonic end is important, especially as it shews 

 the affinity of the modes of development of Osseous fishes 

 and the Elasmobranchii. Oellacher (Zeitschrift filr Wiss. Zoo- 

 logie, Vol. XXXIII. 1873) has shewn, and, though differing from 

 him on many other points, on this point Gotte (Arch, fur Micr. 

 Anat. Vol. IX. 1873) agrees with him, that a similar absence of 

 symmetry by which the embryonic end of the germ is marked 

 off, occurs almost immediately after the end of segmentation 

 in Osseous fishes. In the early stages of development there are 



