94 



EARLY EMBRYOLOGY OF THE CHICK 



being in the mid-brain region, the mesencephalon comes 

 to be the most anteriorly located part of the head and the 

 prosencephalon and myelencephalon lie opposite each other, 

 ventral surface to ventral surface (Fig. 34). The original an- 

 terior end oi the prosencephalon is thus brought in close 

 proximity to the heart, and the optic vesicles and the auditory 

 vesicles are brought opposite each other at nearly the same 

 antero-posterior level. 



mesencephalon 



metencephalic region 

 dorsal aortic root 

 myelencephalic region 



hyomandobular cleft 

 auditory vesicle — 



aortic arch III 



duct of Cuviei 

 sinus venosus 



margin of 

 ant. int. portal 



lateral mesoderm *- 



marginof amnion 

 lateral body fold 



neural tube 

 • 29th somite 



/ lens 



epiphysis 



choroid fissure 

 prosencephalon 



bulbo-conus 

 arteriosus 



omphalomesenteri* 

 artery 



caudal fold 



Fig. 34. — Dextro-dorsal view ( X 14) of entire embryo of 29 somites (about 

 55 hours incubation). 



At this stage flexion has involved the body farther caudally 

 as well as in the brain region. It is especially marked at 

 about the level of the heart in the region of transition from 

 myelencephalon to spinal cord. Since this is the future neck 

 region of the embryo the flexure at this level is known as the 

 cervical flexure (Fig. 34). 



