62 



EARLY EMBRYOLOGY OF THE CHICK 



between the mesencephalon and rhombencephalon. The 

 rhombencephalic neuromeres, however, remain clearly marked 

 for a considerable period. 



By about S3 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 



cephalic neural crest 

 region of ganglion V 



cephalic neural c re st 



region of ganglion 



VII VIII 



.myelencephalon 



prosencephalon 



opdc vesicle 



?* margin of fore-gut 



j " ■ bulbo-conus 



arteriosus 



omphalomesenteric 

 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 He closely approximated to the superficial ecto- 

 derm, a relatibnship of importance in their later development. 

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

 confluent with the cavity of the prosencephalon (prosocoele) . 

 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 



