STRUCTURE OF THIRTY-THREE HOUR CHICKS 



6l 



the vaiious neuromeric enlargements in the formation of the 

 brain regions. The three anterior neuromeres form the prosen- 

 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 

 border of fore 



omphalomesenteric vein 



rhombencephalon 



lateral mesoderm 



notochord 



sinus rhomboidalis primitive streak 



P/G. 21.:— Dorsal view ( X 14) of an entire chick embryo of 12 somites (about 

 ' ^ 33 hours incubation). 



bencephalon (Fig. 20, B), Anteriorly the interneuromeric 

 constrictions soon disappear except for two; namely, the one* 

 between the prosencephalon and mesencephalon, and the one 



