HEAD-FOLD TO TWELVE SOMITES 97 



the undifferentiated mass of the primitive streak. These condi- 

 tions must be born in mind in the following description. 



The Neural Groove and Folds. Shortly after the formation of 

 the head-fold the center of the medullary plate becomes sunk in 

 the form of a deep groove beginning a short distance behind the 



snedpl 



FIG. 51. Embryo of 4 s from above, drawn in alcohol by reflected light, 

 a. c. v., Amnio-cardiac vesicle, a. p., Area pellucida. a. v. i., Inter- 

 nal vitelline area. med. pi., Medullary plate, n. F., Neural fold. Pr'a., 

 Proamnion. pr. str., Primitive streak, s. 1, s. 3, First and third 

 somites. 



