744 



GENERATION AND DEVELOPMENT. 



Immediately beneath, and in a line parallel with the 

 primitive groove, may be seen, about the same time, a 

 narrow linear mass of cells, the chorda dorsalis, which 

 forms the basis around which the bodies of the vertebrae 

 are developed. The development of this column is early 



Fig. 217.* 



indicated by the appearance of a few square, at first in- 

 distinct, plates, the rudiments of vertebrae (fig. 217, D), 

 which begin to appear at about the middle of each dorsal 

 lamina. 



* 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 

 r,, 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 pellucid 

 nerve-substance. Along the bottom of the groove is observed a faint 

 streak, which is probably the chorda dorsalis. D. Vertebral plates 

 (after Bischoff). 



