7O DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



a number of remarkable points of agreement between the 

 Osseous fish and the Dog-fish, combined with a number of 

 equally remarkable 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 

 stages. It appears, however, to have somewhat different re- 

 lations to the blastoderm than the homologous structure in 

 other vertebrates. In its earliest stage which I have observed, 

 it appears as a small cavity in the centre of the lower layer 

 cells. This grows rapidly, and its roof becomes composed 

 of epiblast and only a thin lining of " lower layer " cells, 

 while its floor is formed by the yolk (PL 3, fig. 3, s g}. In 

 the next and third stage (PL 3, fig. 4, s g] its floor is 

 formed by a thin layer of cells, its roof remaining as before. 

 It has, however, become a less conspicuous formation than 

 it was ; and in the last (fourth) stage in which it can be 

 distinguished it is very inconspicuous, and almost filled up 

 by cells. 



What I have called the second stage corresponds to a period 

 in which no trace of the embryo is to be seen. In the third 

 stage the embryonic end of the blastoderm projects outwards 

 to form a structure which I shall speak of as the " embryonic 

 rim," and in the fourth and last stage a distinct medullary 

 groove is formed. For a considerable period during the second 

 stage the segmentation cavity remains of about the same size ; 

 during the third stage it begins to be encroached upon, and 

 becomes smaller both absolutely, and relatively to the increased 

 size of the germ. 



