296 DEVELOPMENT OF ELASMOBRANCH FISHES. 



this it results that that part of the edge of the blastoderm 

 where the embryo is attached forms a bay in the otherwise 

 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 (PI. 9, fig. i). 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. 9, 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. Through- 

 out all this period the edge of the blastoderm has remained 

 thickened, a feature which persists till the complete investment 

 of the yolk, which takes place 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 (PI. 9, fig. 2 y.), 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 Elasmo- 

 branchs 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 possibility of a 

 change in position of this structure is peculiarly interesting, in 

 that it possibly serves to explain how the blastopore of different 

 animals corresponds in different cases with the anus or the 

 mouth, and has not always a fixed situation 1 . 



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, 1875. 

 [This Edition, No. VI.] 



