NOTES ON ELASMOBEANOH DEVELOPMENT. 239 



canalj and is dorsal; this is continuous round the end of the 

 tail, b c, with a ventral part, which extends forwards along the 

 ventral side of the tail, c d, as far as the yolk-stalk, along which 

 it passes, d e, to continue backwards along the yolk, ef, as the 

 slit-like non-embryonic part of the blastopore, which passes 

 behind into the more dilated and posterior part of the so-called 

 yolk-blastopore. 



Shortly after this stage the blastopore completely closes, 

 excepting one point in its dorsal portion, which persists for 

 some time as the neurenteric canal. 



Balfour, as is well known, was the first to compare the primi- 

 tive streak of the Amniota to " the linear streak in Elasmo- 

 branchii, formed by the coalesced edges of the blastoderm 

 which connect the hinder end of the embryo with the still 

 open yolk-blastopore" (' Comparative Embryology,'^ 1st ed., 

 vol. ii, ch. xi, p. 240) ; and he also says, in the same place, 

 that "the passage at the front end of the primitive streak [the 

 neurenteric canal] is the dorsal part of the blastopore, which 

 in Elasmobranchii becomes converted into the neurenteric 

 canal." But he never, either in the chapter quoted or in 

 his account of the actual development of Elasmobranchs in 

 ch. iii, p. 53, describes the ventral embryonic part of the 

 blastopore (woodcut, c d) which connects together the linear 

 streak on the yolk, e/, with the dorsal part of the blastopore, a h? 

 In fact, he says (ch. iii, p. 52), " It is interesting to notice that, 

 owing to the large size of the yolk in Elasmobranchs, the pos- 

 terior part of the primitive blastopore becomes encircled by 

 the medullary folds and tail swellings, and is so closed long 



1 Mem. Ed., p. 288. 



2 This part of the blastopore is clearly recognised and figured by Schwarz. 



