298 



THE EVOLUTION OF MAN. 



spoken of as the "first germ -rudiment or embryonic rudi- 

 ment," as the " primitive germ/' or " the first trace of the 

 embryo." But these designations, which are based on the 

 authority of Baer and Bischoff, are incorrect. For in reality 

 the germ or embryo already exists in the parent-cell, in the 

 Gastrula, and in all the subsequent germ-stages. The germ- 

 shield is merely the earliest rudiment of that dorsal part 

 which first becomes defined. 



Fig. 85.— Germ-area or germ-disc of a Rabbit, with a sole-shaped germ, 

 shield (about ten times enlarged). The light, circular tract (d) is the dark 

 area (a. opaca). The light area (a. pellucida) (c) is lyre-shaped, as is the 

 germ-shield itself (b). Along its axis the dorsal furrow or spinal furrow (a) 

 is seen. (After Bischoff.) 



Fig. 86.— Sole-shaped germ-shield of a Dog ("double shield" of Remak, 

 "embryo-rudiment " of other authors). In the centre is the dorsal furrow, 

 on either side are the spinal swellings, or medullary swellings. 



Fig. 87.— Sole-shaped germ-shield of Chick. 



, After the oval germ-shield has become distinctly defined, 

 in the centre of the light germ-area, along its central line 

 a delicate, white streak appears, which soon becomes pro- 



