EMBRYO OF SEVENTEEN SEGMENTS 



45 



icles to form the lens placode of the e ve (Fig. 43). The^opticjesiVIe is flattened. 

 at this point and will soon invaginate to produce the innerj_n£no3iisJiayer,Qf_the_ 



retina. In the hind-brain region, dorsq;;late rally the ectoderm is thickened and 

 inva ginat ed as the auditory placode (Fig. 45). This placode later forms the 



Optic vesiclt 



Paired vevlral aorta 



Ventral' aorla 



Bulbils cordis 



Ventricle 



Splanchnic mesoderm 



Fovea cardiaca — ir^" - ■■ — f 



Right descending aorta — ! ^a ' » > 

 Vascular plexus — W^a S^ 



Pharyngeal pouch 1 

 Descending aorta 



Pharyngeal pouch Z 



Somatopleure 



Left vitelline vein 

 Entoderm 



Section medullary tube 



Mesodermal 



Splanchnic mesoderm 



Somatopleure 

 Descending aorta 



Medullary tube 

 Splanchnic mesoderm 



Capillary plexus 



__^ — Somatopleure 



Neural groove 



Fig. 41. — Ventral reconstruction of a thirty-eight-hour chick embryo. The entoderm has been removed 

 save about and caudal to the fovea cardiaca. X 38. 



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

 ear (membranous labyrinth). 



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

 yolk caudal to the fovea cardiaca. In Fig. 41 the greater part of the entoderm 

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



