DEVELOPMENT OF ELASMOBRANCH FISHES. 81 



lively stationary, while the growth elsewhere is very rapid. 

 From this it results that that part of the edge of the blas- 

 toderm where the embrj^o is attached forms a bay in the other- 

 wise regular outline of the edge of the blastoderm. By the 

 time that one-half of the yolk is enclosed the bay is a very con- 

 spicuous feature (PL viii. fig. 1). In this figure bl. points to 

 the blastoderm, and yk. to the part of the yolk not yet enclosed 

 by the blastoderm. 



Shortly subsequent to this the bay becomes obliterated by 

 its two sides coming together and coalescing, and the embryo 

 ceases to lie at the edge of the yolk. 



This stage is represented on PI. viil. fig. 2. In this figure 

 there is only a small patch of yolk not yet enclosed {yk)^ 

 which is situated at some little distance behind the embryo. 

 Throughout all this period the edge of the blastoderm has 

 remained thickened, a feature which persists till the complete 

 investment of the yolk, which takes places shortly after the stage 

 last figured. In this thickened edge a circular vein arises, which 

 brings back the blood from the yolk-sac to the embryo. The 

 opening in the blastoderm (PL Vlll. fig. 2 yk.), exposing the 

 portion of the yolk not yet enclosed, may be conveniently called 

 the blastopore, according to Professor Lankester's nomenclature. 



The interesting feature which characterizes the blastopore 

 in Elasmobranchs is the fact of its not corresponding in position 

 with the opening of the anus of Rusconi. We thus have in 

 Elasmobranchs two structures, each of which corresponds in part 

 with the single structure in Amphioxus which may be called 

 either blastopore or anus of Rusconi, which yet do not in Elas- 

 mobranchs coincide in position. It is the blastopore of Elas- 

 mobranchs which has undergone a change of position, owing 

 to the unequal growth of the blastoderm ; while the anus of 

 Rusconi retains its normal situation. In Osseous Fishes the 

 blastopore undergoes a similar change of position. The possi- 

 bility of a change in position of this structure is peculiarly 

 interesting, in that it possibly serves to explain how the blasto- 

 pore of different animals corresponds in different cases with the 

 anus or the mouth, and has not always a fixed situation \ 



1 For a fuller discussion of this question vide Self ,' A comparison of the early 

 stages of development in vertebrates.' Quart. Journ. of Micr. Science, July, 187 j. 



B. 6 



