WEISMANN'S THEORY OF THE GERMPLASM 61 



and Wilson upon the early stages of segmentation 

 of the egg. In the cases of an echinoid and of 

 amphioxus (Fig. 4) they succeeded in shaking apart 

 the first two and the first four cells that arose in 

 division of the egg ; and they traced the subse- 

 quent development of these separated segmentation 

 spheres. 



From one of the first two segmentation spheres 

 of an echinoid egg, Driesch was able to rear suc- 

 cessive embryonic stages (Gastrula and Pluteus), 

 which were normal in shape, but one-half the usual 

 size. Wilson's results, obtained by shaking apart 



4. NOKMAL AND FRACTIONAL GASTPOJL.E AMPHIOXUS. 



(After Wilson.} 



A Gastrula from a whole egg ; B, c and D, gastruLe from single 

 cells artificially separated, (B) from the two-celled stage, (c) from 

 the four-celled, and (D) from the eight-celled stages of normal 

 development. 



the segmentation spheres, were even more interest- 

 ing, as they were performed upon amphioxus, a more 

 highly-organized animal. He reared gastrulss and 

 older embryos with notochord and nerve - tube, 

 which were perfect and normal, except in size. 

 They were one-half, one-quarter, or one-eighth of 

 the usual size, according as they were reared from 



