108 THE SECOND DAY. [CHAP. V. 



1. The second and third cerebral vesicles make 

 their appearance behind the first. 



2. The optic vesicles spring as hollow buds from 

 the lateral, and the unpaired commencement of the cere- 

 bral hemispheres from the front, portions of the first 

 cerebral vesicle. 



3. The auditory plate becomes converted into a 

 pit, opening at the side of the hind-brain or third cere- 

 bral vesicle. 



4. The first indications of the cranial flexure be- 

 come visible. 



5. The head-fold, and especially the splanchno- 

 pleure moiety, advances rapidly backwards ; the head of 

 the embryo is in consequence more definitely formed. 

 The tail-fold also becomes distinct. 



6. The curvature of the heart increases; the first 

 rudiments of the auricles appear. 



7. The circulation of the yolk-sac is established. 



8. The amnion grows rapidly, and the ailantois 

 commences to be formed. 



