B 



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

 overarched and there is the beginning of the proctodeum or anal invagi- 

 nation. In B the proctodeum has met and fused with an evagination of 

 the archenteron. A, anus ; FB, forebrain ; HB, hindbrain ; LV, liver 

 diverticulum; MB, midbrain ; N, notochord ; NT, neurenteric canal ; 

 PD, proctodeum ; PH, pharynx; PN, pineal body; FT, pituitary body. 

 (From Morgan, after Marshall.) 



105 



