KMBRYO OF SEVENTEEN SEGMENTS 



45 



the much thicker wall of the fore-brain. The lateral expansions of the forebrain are the 

 optic vesicles, which eventually give rise to the retina of the eye. The two ectodermal 

 layers are in contact with each other except in the mid-ventral region, where the mesen- 

 chyma is beginning to penetrate between and separate them. The proamnion consists 

 merely of a layer of ectoderm and of entoderm. 



CHICK EMBRYO OF SEVENTEEN PRIMITIVE SEGMENTS (THIRTY-EIGHT HOURS) 



The long axis of this embryo is nearly straight (Fig. 40), the area 



pellucida is dumb-bell shaped and the vascular network is well differenti- 





- 



Vitelline rein 



Proamnion 



Optic vesicle 



Free portion of head 



1 Heart 

 Neural tube 



Notochord 



Rhomboidal sinus 



Primitive streak 



FIG. 40. View of the dorsal surface of a thirty-eight-hour chick embryo. X 20. 



ated throughout the area opaca. The tubular heart is bent to the embryo's 

 right, and opposite its posterior end the vascular network converges and 

 becomes continuous with the trunks of the vitelline veins. Connections 

 have also been formed between the descending aortas and the vascular 

 area, but as yet the vitelline arteries have not appeared as distinct trunks. 



