352 



GENERAL EMBRYOLOGY 



well. In the foregoing pages we have seen that where a germ 

 disc is formed, its margin, known as the germ ring, and recog- 

 nized as the homolog of the lip or margin of the blastopore, is of 

 great importance in the formation of the primary rudiments of 

 the embryo. 



The His- Whitman theory of concrescence emphasizes the 

 general importance and significance of this relation. First 

 stated fully by His, in 1876, the essential idea of this theory was 

 that each side of the germ ring, not only forms, but really is, 



FIG. 162. Diagrams illustrating four stages in the formation of the Tel cost 

 embryo and the growth of the germ ring around the yolk mass. After O. Hert- 

 wig. e, embryo; gr, posterior margin of the germ ring; y, yolk mass; 1, 2, 3, 4, 

 successive positions occupied by the germ ring as it advances over the yolk. 



the rudiment of the corresponding half of the embryo, which is 

 thus actually formed by the approach and gradual, continued 

 fusion posteriorly of the germ ring. In each half of the ring 

 the essential rudiments of the embryo were thought to be al- 

 ready formed, partly at least, and the process of embryo forma- 

 tion consisted merely or chiefly in the junction or addition of 

 these two originally separate halves. The anterior end of the 

 embryo would thus be formed first, and embryo formation 



