64 



THE FIRST DAT. 



Fio. 22. 



[chap. 



Surface View op the Pellucid Area of a Blastoderm of 

 18 hours. 



None of the opaque area is shewn, the pear-shaped outline 

 indicating the limits of the pellucid area. 



At the hinder part of the area isseen the prinaitive groove 

 pr., with its nearly parallel walls, fading away behind, but curv- 

 ing round and meeting in front so as to form a distinct anterior 

 termination to the groove, about half way up the pellucid area. 



Above the primitive groove is seen the medullary groove m.c, 

 with the medullary folds A. These diverging behind, slope away 

 on either side of the primitive groove, while in front they curve 

 round and meet each other close upon a curved line which repre- 

 sents the head-fold. 



The second curved line in front of and concentric with the 

 first is the commencing; fold of the amnion. 



