DEVELOPMENT OF THE ELASMOBRANCH FISHES. 71 



The segmentation cavity of the Dog-fish most nearly agrees 

 with that of Osseous fishes in its mode of formation and re- 

 lation to the embryo. 



Dog-fish resemble Osseous fish in the fact that their em- 

 bryos are entirely formed from a portion of the germ which 

 does not form part of the roof of the segmentation cavity, so 

 that the cells forming the roof of the segmentation cavity 

 take no share at any time in the formation of their embryos. 

 They further agree with Osseous fish (always supposing that 

 the descriptions of Oellacher, loc. cit., and Gotte, Archiv. fur 

 Micr. Anat. Bd. IX. are correct) in the floor of the segmen- 

 tation cavity being formed at one period by yolk. Toge- 

 ther with these points of similarity there are some important 

 differences. 



(1) The segmentation cavity in the Osseous fish from the 

 first arises as a cavity between the yolk and the blastoderm, and 

 its floor is never at any period covered with cells. In the Dog- 

 fish, as we have said above, both in the earlier and later periods 

 the floor is covered with cells. 



(2) The roof in the Dog-fish is invariably formed by the 

 epiblast and a row of flattened lower layer cells. 



According to both Gotte and Oellacher the roof of the 

 segmentation cavity in Osseous fishes is in the earlier stages 

 formed alone of the two layers which correspond with the 

 single layer forming the epiblast in the Dog-fish. In Osseous 

 fishes it is very difficult to distinguish the various layers, 

 owing to the similarity of their component cells. In Dog- 

 fish this is very easy, owing to the great distinctness of the 

 epiblast, and it appears to me, on this account, very probable 

 that the two above-named observers may be in error as to 

 the constitution of its roof in the Osseous fish. With both 

 the Bird and the Frog the segmentation cavity of the Dog- 

 fish has some points of agreement, and some points of differ- 

 ence, but it would take me too far from my present subject to 

 discuss them. 



When the segmentation cavity is first formed, no great 

 changes have taken place in the cells forming the blastoderm. 

 The upper layer the epiblast is composed of a single layer 

 of columnar cells, and the remainder of the cells of blastoderm, 



