ON THE DEVELOPMENT OF THE ELASMOBKANCH FISHES. 333 



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 dif- 

 ference, 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 blasto- 

 derm, forming the lower layer, are of a fairly uniform size, 

 and polygonal from mutual pressure. The whole edge of 

 the blastoderm is thickened, but this thickening is especially 

 marked at its embryonic end. 



This thickened edge of the blastoderm is still more con- 

 spicuous in the next and second stage (PI. XIII, fig. 3). 



In the second stage the chief points of progress, in addition 

 to the increased thickness of the edge of the blastoderm, 

 are — 



(1) The increased thickness and distinctness of the epi- 

 plast, caused by its cells becoming more columnar, though it 

 remains as a one-cell-thick layer. 



(2) The disappearance of the cells from the floor of the 

 segmentation cavity. 



The lower layer cells have undergone no important 

 changes, and the blastoderm has increased very little if at 

 all in size. 



From PI. XIII, fig. 3, it is seen that there is a far larger 

 collection of cells at the embryonic than at the opposite 

 end. 



Passing over some rather unimportant stages, I will come 

 to the next important one. 



The general features of this (the third) stage in a surface 

 view are — 



(1) The increase in size of the blastoderm. 



(2) The diminution in size of the segmentation cavity, 

 both relatively and absolutely. 



(3) The appearance of a portion of the blastoderm pro- 

 jecting beyond the rest over the yolk. This projecting rim 

 extends for nearly half the circumference of the yolk, but is 



