DEVELOP*] 







\VI.,M. tli.- lad i. -in!. ant of the |c groove has been employed 



for the elongation of the primitive -;n>ov., th.- margin of theg 

 disc, uhii-h continues all th,- timeto spread Itself ou1 uniformly 

 tin- v. ,lk, exhibits -\-.-:-yuhore one tu.d tin- Bame condition; it has 

 become .-it .-ill p..int> a oircwnciwc yin, n.w that the 



orpin has detached itself from it as primitive groove. 



Fig. bo 



Fig. 86. 



Tig. 5.- Surface view of the area pellucida in the blastoderm of a Chick, goon after the 

 foimation of the primitive groove, after BAI i 



; r, I'limitive streak with primitive groove; //, :inini"t:c t\M. Tin- <l.ukT .-h.ulni^ >ui i> 

 tho ]iriinitive stre.-ik indicates the extent of the inesublast. 



Fig. 86. Surface view of the area pellucida of a blastoderm of 18 hour*, after BALFOUR. 



Tlu-.uv;ii.| a. . tin- ii-;ir-.-h;ijK'il outline in;uks the limit of the area {Hjllucula. At the 



jilaiv \\h<-i e :),. tu.. iiifinliary { . LIU.-, with fiii-li i.thi-r tliere is to be seen a 



short cur\eii line, which represents the hejul-f"!.!. In front of it tlieie lies a seou,.l line 



uc with it, the b-xinn.ng of the amniutic foil. A, Medullary folds ; 

 furrow ; j>r, jirimitive groove. 



When subsequently the pellucid and opaque areas become more li- 

 tim-tly si-pa i sited, the primitive groove comes to lie in the posterior 

 part of the pellucid area. By caivt'nl cxamii.ation of a surface pre- 

 paration (figs. 85 and 80 ;), one sees that it is l>om:.lt>d, both on the 

 riirlit >ule and on the left, by two small folds, which are derived 

 from the blastoporic lips, and which appeal- darker ai'd more opaque 

 because the cells are multiplying rapidly and are more closely 

 crowded. Since the two primitive folds, or the two blastoporic lips r 



