ON THE DEVELOPMENT OF THE ELASMOBRANCH FISHES. 331 



and can be distinguished from the remainder of the cells of 

 the blastoderm as a separate layer. This layer forms the epi- 

 blast; 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 blas- 

 toderm between the end at which the embryo will appear, 

 and the non-embryonic end is important, especially as 

 showing the affinity of the modes of development of Osseous 

 fishes and the Elasmobranchii. Oellacher (' Zeitschrift fiir 

 Wiss. Zoologie,' vol. xxxiii, 1873) has shown, and, though 

 differing from him on many other points, on this point 

 Gotte (' Arch, fiir 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 develoj)ment there are a number of 

 remarkable points of agreement between the Osseous fish 

 and the Dog-fish, combined with a number of equally remark- 

 able points of difference. Some of these I shall point out as 

 I proceed with my description. 



The embryonic end of the germ is always the one which 

 points towards the pole of the yolk farthest removed from 

 the egg-capsule. 



The germ grows, but not very rapidly, and without other- 

 wise undergoing any very appreciable change, for some time. 



The growth at these early periods appears to be particularly 

 slow, especially when compared with the rapid manner in 

 which some of the later stages of the development are passed 

 through. 



The next important change which occurs is the formation 

 of the so-called " segmentation cavity." 



This forms a very marked feature throughout the early 



