344 



THE EVOLUTION OF MAN. 



medullary tube, the two primitive vertebral cords in the 

 centre of the primitive germ break up into a number of 

 pieces, lying one behind another, and resembling small 

 cubes on each side of the medullary tube. Two pairs 

 usually first make their appearance simultaneously. Then 



Fig. 106-109.— The Germ- 

 disc of a Eabbit (the circular 

 germ -area with the lyre- 

 shaped germ -shield), seen 

 from the dorsal surface, in 

 four consecutive stages of 

 evolution (about ten times 

 the natural size). (After 

 Bischoff.) 



In Fig. 106 the embryo (b) 

 is as yet without primitive 

 vertebrae ; the open dorsal 

 furrow (a) surrounded by 

 a narrow light germ-area 

 (a. pellucida, a), in the 

 middle of the dark germ- 

 area (a. opaca, d). 



In Fig. 107 seven pri. 

 mitive vertebrae (c) may 

 already be seen; the dorsal 

 furrow is closed; the first 

 rudiment of the brain (a), 

 a brain -bladder, behind 

 which a second (&) is form- 

 ing, is arising; the light 

 germ -area is now only 

 visible at the anterior end, 

 in the form of a dark sickle- 

 shaped body on a black 

 ground. 



