MISCELLANEOUS NOTES. 



155 



embryonic area. The edges of the groove (or track left in the wake of 

 the yolk-plug) must therefore be considered the lateral blastopore lips. 

 Fig. 2, A, B, C will make this clear. A is the stage in which the yolk- 

 plug has not yet begun its backward movement ; in B it has moved 

 some distance, and in G it has reached its final position. In tbe last 

 stage, the blastopore therefore consists of three parts : — 



Fig. 2. 



A 



Yk. Flag. 



Diagrams showing three successive stages in the development of the posterior part of 

 the embryo. 



(1) the dorsal opening of the neurenteric canal. 



(2) the groove between ( 1 ) and (3). 



(3) the groove around tbe yolk-plug in its final position. 



