DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 



When the last remnant of the crescentic groove has been employed 

 tor the elongation of the primitive groove, the margin of the germ- 

 disc, which continues all the time to spread itself out uniformly over 

 the yolk, exhibits everywhere one and the same condition; it has 

 become at all points a circumcrescence-margin, now that the in- 

 vagination-margin has detached itself from it as primitive groove. 



Fig. 6i 



Fig. 86. 



Fig. 85. Surface view of the area pellucida in the blastoderm of a Chick, soon after the 



formation of the primitive groove, after BALFOUK. 

 pr, Primitive streak with primitive groove ; a/, amniotic fold. The darker shading surrounding 



the primitive streak indicates the extent of the mesoblast. 



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



The area opaca is omitted ; the pear-shaped outline marks the limit of the area pellucida. At the 



place where the two medullary folds are continuous with each other there is to be seen a. 



short curved line, which represents the head-fold. In front of it there lies a second line 



concentric with it, the beginning of the amniotic fold. A, Medullary folds ; me, medullary 



furrow ; pr t primitive groove. 



When subsequently the pellucid and opaque areas become more dis- 

 tinctly separated, the primitive groove comes to lie in the posterior 

 part of the pellucid area. By careful examination of a surface pre- 

 paration (figs. 85 and 86 pr), one sees that it is bounded, both on the 

 right side and on the left, by two small folds, which are derived 

 from the blastoporic lips, and which appear darker and more opaque 

 because the cells are multiplying rapidly and are more closely 

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



