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 
DCE - 
wi 
sie 
iB 
Fic. 51. — Embryo of 4s from above, drawn in alcohol by reflected light. 
a. c. v., Amnio-cardiae vesicle. a.p., Area pellucida. a. v.i., Inter- 
nal vitelline area. med. pl., Medullary plate. n.F., Neural fold. Pr’a 
Proamnion. pr. str., Primitive streak. s. 1, s. 3, First and third 
oe) 
somites. 
