NOTES ON ELASMOBRANCH DEVELOPMENT. 563 
gut, and the two slits which are continuous with one another 
round the hind end of the embryo are portions of the blasto- 
pore. By the timethat the two angles marked a and the edges of 
the embryonic part of the blastoderm have come into contact 
ventrally, the non-embryonic edges of the blastoderm adjacent 
to the embryo have grown backwards over the yolk to form 
the bay mentioned by Balfour. The two sides of this bay, 
which it will be remembered are portions of the edges of the 
blastoderm, come to lie close together on the yolk beneath 
the tail of the embryo. For a little time they remain unfused, 
and the yolk is still freely exposed between them in a linear 
streak.! This slit, which is bounded by the edges of the non- 
embryonic part of the blastoderm of the two sides, is a part of 
the blastopore, and is continuous, passing along the hinder 
side of what will be called the umbilical stalk, with the portion 
of the blastopore leading into the hind gut and extending 
along the ventral side of the tail. This last portion is, as we 
have seen, continuous with a dorsal portion which leads 
through the medullary plate into the medullary canal. 
The last part of the blastopore to be mentioned is the so- 
called yolk-blastopore, described by Balfour in the ‘ Elasmo- 
branch Fishes,’ p. 81 (Mem. Edition, vol. iii, p. 296), and in 
the ‘ Comparative Embryology,’ Ist ed., ch. iii, p. 52.2. The 
lips of this portion are continuous with the lips last men- 
tioned as running back on the yolk parallel to one another, 
and ventral to the tail of the embryo. 
To recapitulate: the blastopore of Hlasmobranchii is at the 
present stage—i. e. the stage immediately before closing—an 
elongated narrow slit, slightly dilated in front, where it hes 
on the floor of the medullary canal (fig. 3) and more dilated 
behind (Balfour’s yolk-blastopore, ‘ Comparative Embryology,’ 
vol. ii, ch. iui, fig. 30 6). Between these two limits it takes 
the course of a reversed letter S, as shown in the adjoining 
woodcut, where its lips are represented unfused. 
The anterior part, a J, perforates the floor of the medullary 
1 Again see Schwarz’s fig. 16, d.o. 
2 Mem. Ed., vol. iu, p. 63. 
