NOTES ON ELASMOBRANOH DEVELOPMENT. 241 



to the yolk-stalk (my woodcut, c 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, abed) 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- 



