HEAD-FOLD TO TWELA E 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 





"■;*-■■■"'■ 





"-'.>; j/.-.-Tj. *;-"'.--■;?••■ ■.'•'.'.■.■■ -j P'/"-^'!-t.' '"■■'•■.;■ .'.T.-':'-' '-.'■• '.' ■'-. '■■■ '.v'V- ■':;'.'- '.•'•» ''-". '■'■''- ■■■ "■ ' .■ >'':'^- ::■':" '■ 

 ; ■■ . .•■'-V.-.. ■:•.-•■ ■■'■'', :'^^:■'•.■-^''■^S•v'.■■ ..:-';'■ 7. ';■-•■■•. ■-:-»■• • v.\-*^v jfyi :<7l'J^-^i--^. '^>' ;.'i-- ;:-'i .■.-■'• ". •..-,; . ;■ -■ ',' ■■■•:■ 



^ ■'■■- -■■ .. ■" .■■;--;;\v':r ^t^v-^-;-v.v. •'!•.■• -.•..'s-'^.^'.' jt> ~^t/ pA: r.z^y. xv ■ V.'-. ■•Viv^;,-..-'; .-,.':.■. -:.:v,v ..'..'■ v.-'::-,/; 





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. pL, Medullary plate, n. F., Neural fold. Pr'a., 

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

 somites. 



