LV PH 



Fig. 25.— Sagittal sections through two embryos. In A the blastopore is 

 overarched and there is the beginning of the proctoda'lun or anal invagi- 

 nation. In B the proctoda-uni has met and fused with an evagination of 

 the archenleron, ^, anus ; Z-'^, forcbrain ; //Z/, hindbrain ; Z./' liver 

 d.vert.culum ; ^/Z^, midbrain ; A^, notochord ; A'r. neurenteric canal; 



% P™*;*"^^""^ : P^^' pharynx; PN, pineal body; PT, pituitary body. 

 (From Morgan, after Marsh:Jl.) ^ i 3 



105 



