220 Vertebrate Embryology 



intermediate cell-mass, and may even project, 

 somewhat, into the body-cavity (Fig. 71, CIV). 



The final fate of the Wolffian body, as well 

 as the development of the pronephros and the 

 metanephros, will be taken up later. 



Summary of the third day : 



1. Turning of the embryo to lie on its left 

 side. 



2. Increase of cranial flexure, and begin- 

 ning of body flexure. 



3. The formation of the four gill clefts and 

 the five gill arches. 



4. The completion of the circulation of the 

 vascular area ; the increased curvature of 

 the heart, and the indication of its division 

 into chambers ; the appearance of new aortic 

 arches, of the cardinal veins, and of the meatus 

 venosus. 



5. The formation of the optic cup. 



6. The formation of the lens vesicle. 



7. The formation of the nasal pits. 



8. The closing in of the auditory vesicles. 



9. The first indication of the cerebral hem- 

 ispheres, and the separation of the hind-brain 

 into cerebellum and medulla oblongata. 



10. The definite establishment of the cra- 

 nial and spinal nerves. 



