BLASTULA, GASTRULA, AND GERM LAYERS 353 



would be complete when the germ ring became fully contracted 

 or closed. 



With some modifications of a really fundamental kind, this 



D 



FIG. 163. Diagrams of the formation of an embryo by confluence ("con- 

 crescence"). A. Germ ring before formation of the embryo is indicated. The 

 letters a-e, represent symmetrical portions of the germ ring. B. Beginning of 

 confluence. C. Embryo forming. A A, BB, represent regions of the embryo, 

 formed out of the materials of the germ ring at aa, bb. D, E, Later stages in the 

 formation of the embryo. The germ ring regions, cc, and dd, have been differ- 

 entiated into the embryonic regions, CC, DD. 



conception is widely adopted to-day. Both observation and 

 experiment have shown, however, that definite halves of an 

 embryo cannot be said to exist preformed in the lateral portions 

 of the germ ring. These regions do contribute to the formation 



