564 ADAM SEDGWICK. 
canal, and is dorsal; this is continuous round the end of the 
tail, 6c, with a ventral part, which extends forwards along the 
ventral side of the tail, ¢ 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,’! Ist 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. 52, describes the ventral embryonic part of the 
blastopore (woodcut, c d) which connects together the linear 
streak on the yolk, e f, with the dorsal part of the blastopore, a 6.” 
In fact, he says (ch. ii, 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. 
