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 of 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 re 

 myelencephalic region 



hyomandibular cleft 

 auditory vesicle 



aortic arch III 



duct of Cuvier 

 sinus venosus 



margin of 



ant. int. Dortal 



optic cup 

 , lens 



epiphysis 



choroid fissure 



prosencephalon. 



bulbo-conus 

 arteriosus 

 atrium 



V^ 



Hfc- . atrium 



#- >i , ventricle 



lateral mesoderm 

 marginof amnion 

 lateral body fold 



neural tube 

 29th somite 



omphalomesenteri* 

 artery 



audal 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). 



