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 



f:-:>-^ 





^ 







•/^2. ' 



'■m. 











f '-! -. - ■-. ■ *^',-,"rr--^, ^-■— : X ■, ■■ '. , '■ ■; '^ir- .■-••-vf ,^^.';- ----■ ■*<-*r-ti.-=^'--o-vv- --i-J-S- tT-^-"'- --iv'i''- r-'*i>'-3 - ■-■■■.:-•'- 



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., 

 l^roamnion. pr. str., Primitive streak, s. 1, s. 3, First and third 

 somites. 



