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. 



Fig. 90. Germ-disc of an embryo Rabbit -with primitive streak, after E. VAN BEXEDEW. 



pr, Primitive streak ; Tcf, head-process ; hk, HEXSEN'S node ; en, canalis neurentericus. 



Fig. 91.-An embryo Rabbit with a part of the area pellucida 9 days after fertilisation. 

 Magnified 22 diameters. After KO'LLIKER. 



ap, Area pellucid* ; ao, area opaca ; A', medullary plate in the region of subsequent first brain- 

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

 (rf) exhibits a widening ; h'", the same in the region of the subsequent third brain- 

 vesicle ; fcz, fundament of the heart ; stz, trunk zone (Stammzone) ; pz, parietal zone ; pr, 

 remnant of the primitive streak. 



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

 pronounced guitar-shaped outline. Such an embryo is represented 

 in fig. 91. The primitive streak (pr} 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 

 aides of the chorda. 



Just as there has been up to this stage an agreement with .Birds 



