62 



EARLY EMBRYOLOGY OF THE CHICK 



between the mesencephalon and rhombencephalon. The 

 rhombencephalic neuromeres, however, remain clearly marked 

 for a considerable period. 



By about 33 hours of incubation the optic vesicles are estab- 

 lished as paired lateral outgrowths of the prosencephalon. 

 They soon extend to occupy the full width of the head (Fig. 

 20, C and Fig. 21). The distal portion of each of the vesicles 



protencephalon 



optic vesicle 



aortic arch 



notochord 



margin of fore-gut 



ectoderm 



infundibulum 



mesencephalon 



ventral 

 aortic root 



cephalic neural crest 

 region of ganglion V 



bulbo-conus 



arteriosu 



cephalic neural crest 



region of ganglion 



VII VIII 



omphalomeienteric 

 vein 



PIG. 22. Dorsal view ( X 45) of head and heart region of a chick embryo of 17 

 somites (38-39 hours incubation). 



thus comes to lie closely approximated to the superficial ecto- 

 derm, a relationship of importance in their later development. 

 At first the cavities of the optic vesicles (opticceles) are broadly 

 confluent with the cavity of the prosencephalon (prosoccele). 

 Somewhat later constrictions appear which mark more defi- 

 nitely the boundaries between the optic vesicles and the prosen- 

 cephalon (Fig. 20, D and Fig. 22). 



There arises also at this stage a depression in the floor of the 



