CHICK EMBRYO OF EIGHTEEN PRIMITIVE SEGMENTS 



55 



icles to form the lens placode of the eye (Fig. 41). The optic vesicle is flattened 

 at this point and will soon invaginate to produce the inner, nervous layer of the 

 retina. In the hind-brain region, dorso-laterally the ectoderm is thickened and 

 invaginated as the auditory placode (Fig. 43). This placode later forms the 

 otocyst or otic vesicle from which is differentiated the epithelium of the internal 

 ear (membranous labyrinth). 



Farsbrain 

 Mid-brain H 

 Hind -brains. 



Vitelline Vein 



Mesodermal W% 

 seamen/ 3 £ 



Notochord § 



Proamnion 

 Optic vesicle 



Free portion 

 of he-ad 



Heart 



Neural tube 



- - • ■ 



Rhomboidal 

 sinus 



, ' ' , /'■ t Primitive 



Wmm sTreaK 





Fig. 39. — View of the dorsal surface of a thirty-six-hour chick embryo. X 20. 



Digestive Tube. — The entoderm is still flattened out over the surface of the 

 yolk caudal to the fovea cardiaca. In Fig. 40 the greater part of the entoderm 

 is cut away. The flattened fore-gut, folded inward at the fovea, shows indications 

 of three lateral diverticula, the pharyngeal pouches. Cephalad the pharynx is 

 closed ventrally by the pharyngeal membrane. 



