238 APAM SEDGWICK. 



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 time that 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,' 1st ed., ch. iii, p. 52.^ 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 Elasmobranchii is at the 

 present stage — i. e. the stage immediately before closing — an 

 elongated narrow slit, slightly dilated in front, where it lies 

 on the floor of the medullary canal (fig. 3) and more dilated 

 behind (Balfour's yolk-blastopore, ' Comparative Embryology,' 

 vol. ii, ch. iii, fig. 30 b). 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 b, perforates the floor of the medullary 



' Again see Schwarz's fig. 16, d.o. 

 2 Mem. Ed., vol. iii, p. 63. 



