128 



EMBRYOLOGY. 



From this time forward the anterior part of the embryonic area 

 grows in length much more rapidly than the hind part with its 

 primitive groove ; the latter remains almost unaltered in Mammals 

 up to late stages of development, and then diminishes in length, not , 

 only relatively, but also absolutely. 



"'^ >'i'''^:-'?l|tK 



Kg. 90. 



Kg, 91. 



Fig. 90.— Germ-disc of an embryo Sablit with primitive streak, after E. van Benedeu. 

 W, Primitive streak ; Tcf, head-process ; fti, Hessen's node ; en, canalis neuientericus. 



Fig. 91.— An embryo Eabbit with a part of the area pellucida 9 days after fertilisation, 

 Kagnified 22 diameters. After KHlliker. 



ap. Area pellucid i ; ao, area opaoa; *', medullary plate in the region of subsequent first brain- 

 vesicle ; A", the same in the region of the subsequent mid-brain, where the medullary furrow 

 (rf) exhibits a widening; /t'", the same in the region of the subsequent thii-d brain- 

 vesicle ; ftz, fundament of the heart ; stz, trunk zone (Stammzone) ; pz, parietal zone ; J)r, 

 remnant of the primitive streak. 



At the same time the embryonic area passes from the oval to » 

 pronounced guitar-shaped outline. Such an embryo is represented 

 in fig. 91. The primitive streak (p-) is to be seen at its posterior 

 end, partly embraced by the medullary folds (rf). The middle germ- 

 layer is already fully developed, and in the future neck-region three 

 pairs of primitive segments have already been differentiated at the 

 sides of the chorda. 



Just as there has been up to this stage an agreement with Bii-ds 



