iig. 4.— Successive stages of the development of the head of a Chick. 

 I, II, III, first, second, and third cerebral vesicles ; la, vesicle of the 

 cerebral hemisphere ; lb, vesicle of the third ventricle ; a, rudiments 

 of the eyes and optic nerves ; i, of the ears ; g, of the olfactory 



