ORIGIN OF THE PRIMITIVE VERTEBRAE. 



345 



appear three, four, and five pairs, and finally a larger number 

 of these pieces, which are called the primitive vertebrae. 



In Fig. 108 the em- 

 bryo has eight primi- 

 tive vertebrae and 

 three brain-bladders ; 

 the first brain -bladder 

 (b) shows two lateral 

 protuberances, the 

 first rudiments of the 

 eye -bladders (c) ; the 

 second (d) and the 

 third (e) brain-blad- 

 ders are much 

 smaller ; a indicates 

 the edge of the head- 

 sheath of the amnion. 



In Fig. 109 the 

 embryo has ten 

 primitive verte- 

 brae ; in the germ- 

 area the first 

 traces of the net 

 of blood-vessels 

 appear, bounded 

 by the ve7t,a term i- 

 rialis (a) : b, tail- 

 sheath, 6b, head- 

 sheath of the 

 amnion; the folds 

 in the latter indi- 

 cate the serous 

 membrane. 



