DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 



127 



the embryonic area has assumed an oval form, the opacity at the 

 posterior end, or the terminal ridge (fig. 63 hw), which was compared 

 with the crescent of the Bird, elongates into the primitive streak ; 

 the latter occupies the posterior half of the embryonic area (fig. 89 

 A pr), and exhibits a distinct groove, that is flanked by a right and a 

 left ridge-like fold. (Compare with this the Chick as shown in fig. 85.) 



Fig, 89 A. Embryonic fundament of an 8-days Babbit, after KOLI.IKER. 

 arg, Fundament of the embryo ; pr, primitive streak. 



Fig. 89 B. Vascular area (o) and embryonic fundament (.(/) of a 7-days Rabbit's egg, after 



KOLLIKEK. 

 o, Vascular area (area opaca) ; ag, embryonic fundament ; pr, primitive groove ; rf, medullary 



furrow. 



Afterwards there appears in this instance, just as with the Chick, a 

 narrow opaque streak in the forward prolongation of the primitive 

 streak, its head-process, and this divides the anterior portion of 

 the germ into a right and a left half (fig. 90 kf). After some time 

 there are developed on both sides of the head-process the medullary 

 folds (fig. 89 .5), which bound the broad medullary furrow (rf}, and 

 which, by forming a bow at their anterior ends, become continuous 

 with each other ; but posteriorly they diverge somewhat from each 

 other, and embrace the primitive groove (pi'). This stage corresponds 

 to the condition of the Chick presented in fig. 86. 



