16 



TEE ANATOMY OF YERTEBRATED ANIMALS. 



even in this early stage, a clear distinction is drawn between 

 the skull and the spinal column. 



/j^^^^.-r 



pfi 1 § j' lip 



txi. i. — Successive stag'es of the development of the 'head of a Chick. I, II, III, first sec- 

 ond, and third cerebral vesicles ; la, vesicle of the cerebral hemisphere ; lb, vesicle of 

 the thiixl ventricle ; a, rudiments of the eyes and optic nerves ; b, of the ears ; g. of the 

 olfactory organs ; d, the infundibulum ; e. the ])ineal glaud ; c, protovertebrfe ; h. noto- 

 chord; 1, 2, 3, 4, 5, visceral arches; V, VII, VIII, the trigeminal, portio dura, and ninth 

 and tenth pairs of cranial nerves; k, the nasal process; I, the maxillary process; x, tlia 

 first visceral cleft. A. B, upper and under views of the head of a Chick at the end of 

 the second day. C, side-view at the third day. D. side-view at seventy-five hours. E, 

 side-view of the head of a Chick at the fifth day, which has been subjected to shght press- 

 Ore. F, head of a Chick at the sixth day, viewed from below. 



