FATE OP BLASTOPORE, ETC., IX CHELONIA. 



83 



Woodcut XII. 



Woodcut XIII. 



h c, with a ventral part, which extends forwards along tlie ventral 



side of the tail, c d, as far as tVie yolk- 

 stalk, along which it passes, d e. to continue 

 backwards along the yolk, e f, 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." 



Thus far Sedgwick. To make the i-elations brought out by him 

 still clearer, I give here a diagrammatic 

 figure (Woodcut XIII) constructed by my- 

 self from Ziegler's figures and representing 

 the yolk as already covered by the blasto- 

 derm excepting a small area called by 

 Balfour the yolk-blastopore. The blasto- 

 pore is indicated by a curved line extending 

 from a to <j, and has its ditferent parts 

 marked by the same letters as in the figure 

 above given of Sedgwick. That is, a is 

 the neurenteric canal, a h is along the dorsal 

 surface of the tail, h c around the tail end. 



Diagram of an Elasmobranch 



e"iWo. c d along the median ventral line, (/ e down 



the yolk-stalk, e f the coalesced edges of the non-embryonic part 

 of the blastopore, and finally the circular opening, g, exposing the 

 yolk on the surface. 



I must now ask the reader to compare the course of changes 

 recounted above as taking place in the Elasmobranchs with that I 



