566 ADAM SEDGWICK. 
to the yolk-stalk (my woodcut, ¢ d); while the linear part 
of the yolk-blastopore in Elasmobranchii (my woodcut, e f) is 
unrepresented or rudimentary in Aves and Amniota gene- 
rally—is, in fact, the rudimentary part referred to by Balfour 
in the above quotation from the ‘ Comparative Embryology.’ 
The comparison has the advantage of bringing together the 
growing points of the embryos in the two cases. In 
Amniota the primitive streak is the growing point where the 
cells are proliferated, out of which the greater part of the 
embryo is formed. In Elasmobranchii the tail swellings 
which form the sides of the dorsal and ventral parts of the 
embryonic blastopore (my woodcut, a b ¢ d) are the points 
where the active growth takes place, as a result of which the 
hinder part of the embryo is formed. Indeed, the prominence 
of the tail swelling is due to the mass of mesoderm-cells pro- 
duced by this proliferation at the edge of this part of the 
blastopore. 
The proliferation of mesoderm takes place in a rudimentary 
fashion in Elasmobranchii, at all points of the circumference 
of the blastoderm; which circumference, gradually creeping 
over the yolk and enclosing it, constitutes the lips of the 
widely open blastopore; but the proliferation is very feeble 
except at the notched embryonic rim, the growth of which 
forms, as above described, the tail end of the embryo. 
It is interesting to notice the different manner in which the 
tails of Elasmobranchii and Amniota are formed. ‘There is in 
the former no tail fold as in the latter, but simply a bilateral 
bending round of the posterior tongue-like projection formed 
by the growth backwards of the notched part of the embry- 
onic rim. 
The above account of the Elasmobranch blastopore is not 
given for the first time, although when I did my work—now 
some years ago—I was unaware that a correct account of the 
process had been published by Schwarz in 1889 (‘ Zeit. f. wiss. 
Zool.,’ Bd. xlviii). 
Kastschenko, in the previous year, published an excellent 
paper on Selachian development in the ‘Anatomischer An- 
