194 THE THIRD DAY. [CHAP. VI. 



3. The completion of the circulation of the yolk- 

 sac; the increased curvature of the heart, and the 

 demarcation of its several parts; the appearance of new 

 aortic arches, and of the cardinal veins. 



4. The formation of four visceral clefts and five 

 visceral arches. 



5. The involution to form the lens, and the forma- 

 tion of the secondary optic vesicle. 



6. The closing in of the otic vesicle. 



7. The formation of the nasal pits. 



8. The appearance of the vesicles of the cerebral 

 hemispheres ; the separation of the hind-brain into cere- 

 bellum and medulla oblongata. 



9. The definite establishment of the cranial and 

 spinal nerves as outgrowths of the central nervous 

 system. 



10. The completion of the fore-gut and of the 

 hind-gut; the division of the former into oasophagus, 

 stomach and duodenum, of the latter into large intestine 

 and cloaca. 



11. The formation of the lungs from a diverticulum 

 of the alimentary canal immediately in front of the 

 stomach. 



12. The formation of the liver and pancreas: the 

 former as two diverticula from the duodenum, which 

 subsequently become united by nearly solid outgrowths ; 

 the latter as a single diverticulum also from the duo- 

 denum. 



13. The changes in the mesoblastic somites and 

 the appearance of the muscle-plates. 



14. The definite formation of the Wolffian bodies 

 and the change in position of the Wolffian duct. 



