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. 91. 



Fig. 90. Germ-disc of an embryo Babbit with primitive streak, after B. VAN BENEDEN. 

 ?, Primitive streak ; kf, head-process ; Me, 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. 



op, Area pellucid* ; ao, area opaca ; h', 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 ; hz, 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 

 sides of the chorda. 



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



