Majichester Memoirs, Vol. I. (1906), No. 3. 21 



this process is taking place at the dorsal lip, that is, at the 

 median posterior region of the blastoderm's edge, it is also 

 being extended, though in a far less degree, to the neigh- 

 bouring regions, the lateral lips, on the right and left 

 (^Fig. 7, A, B). The archenteron thus comes to assume a 



'^^c^*^ 



Fig. 8. 

 Growth of the Elasmobranch blastoderm over the yolk and final 

 envelopment of the latter ; a.e. anterior edge which becomes v.l. ventral lip ; 

 y.b. yolk-blastopore. In A the median strip of yolk behind the dorsal lip is 

 seen bounded by the two lateral lips. 



crescentic shape, with a median anterior prolongation ; 

 the latter underlies the embryonic portion of the blasto- 

 derm, the crescentic part is wholly extra-embryonic, and 

 remains very shallow, though it is subsequently prolonged 

 to the right and left round the edges of the blastoderm 



