THE UMBILICAL VESICLE. 



583 



strict the yelk, and inclose only a part of it ; and soon after- 

 wards the yelk and the inner layer of the germinal membrane 



FIG. 217. 



Portion of the germinal membrane, with rudiments of the embryo; from the 

 ovum of a bitch. The primitive groove, A, is not yet closed, and at its upper or 

 cephalic end presents three dilatations B, which correspond to the three divisions or 

 vesicles of the brain. At its lower extremity the groove presents a lancet-shaped 

 dilatation (sinus rhomboidalis) c. The margins of the groove consist of clear pel- 

 lucid nerve substance. Along the bottom of the groove is observed a faint streak, 

 which is probably the chorda dorsalis. D. Vertebral plates. After Bischoff. 



FIG. 218. 



Diagram showing vascular area in the chick, a. Area pellucida. 6. Area 

 vasculosa. c. Area vitellina. 



