DEVELOPMENT OF THE ELASMOBRANCH FISHES. 73 



ment I will, however, reserve a discussion of its meaning till 

 the next stage. 



The only other important feature of this stage is the ap- 

 pearance of a layer of cells on the floor of the segmentation 

 cavity. 



Does this layer come from an ingrowth from the thickened 

 edge of the blastoderm, or does it arise from the formation of 

 new cells in the yolk ? 



It is almost impossible to answer this question with cer- 

 tainty. The following facts, however, make me believe that 

 the newly formed cells do play an important part in the forma- 

 tion of this layer. 



(1) The presence at an earlier date of almost a row of nuclei 

 under the floor of the segmentation cavity (PI. 3, fig. i). 



(2) The presence on the floor of the cavity of such large cells 

 as those represented in fig. i, b d, cells which are very different, 

 as far as the size and granules are concerned, from the remain- 

 der of the cells of the blastoderm. 



On the other hand, from this as well as other sections, I 

 have satisfied myself that there is a distinct ingrowth of cells 

 from the embryonic swelling. It is therefore most probable 

 that both these processes, viz. a fresh formation and an ingrowth, 

 have a share in the formation of the layer of cells on the floor 

 of the segmentation cavity. 



In the next stage we find the embryo rising up as a distinct 

 body from the blastoderm, and I shall in future speak of the 

 body, which now becomes distinct as the embryo. It cor- 

 responds with what Kupffer (loc. cit.} in his paper on the 

 "Osseous Fishes" has called the "embryonic keel." This 

 starting-point for speaking of the embryo as a distinct body is 

 purely arbitrary and one merely of convenience. If I wished to 

 fix more correctly upon a period which could be spoken of as 

 marking the commencing formation of the embryo, I should 

 select the time when structures first appear to mark out the 

 portion of the germ from which the embryo becomes formed ; 

 this period would be in the Elasmobranchii, as in the Osseous 

 fish, at the termination of segmentation, when the want of sym- 

 metry between the embryonic end of the germ and the opposite 

 end first appears. 



