STRUCTURE OF THIRTY-THREE HOUR CHICKS 



6l 



the vaiious neuromeric enlargements in the formation of the 

 brain regions. The three anterior neuromeres form the proseri- 

 cephalon; neuromeres four and five are incorporated in the 

 mesencephalon ; and neuromeres six to eleven in the rhom- 



prosencephalon 



proamnion 



anterior neuropore 



optic vesicle 



mesencephalon 



heart 



border of f 



rhombencephalon 



ic vein 



otochord 



neural tube 



sinus rhomboidalis primitive streak 



PIG. 21. Dorsal view ( X 14) of an entire chick embryo of 12 somites (about 

 33 hours incubation). 



be.ncephalon (Fig. 20, B). Anteriorly the interneuromeric 

 constrictions soon disappear except for two; namely, the one 

 between the prosencephalon and mesencephalon, and the one 



