Experiment 

 edeggs tin 





third process in the reorganization-, namelv the <jro\vth of 

 cells from the uninjured half over the injured half or at least 

 over such parts of it as present vacillated yolk. All three pro- 

 cesses may take place at once or separately. 



Post generation is the completion of the half of the embryo 

 in which the above nuclear changes have regenerated the mass 

 apparently killed by the injury of operation. It is externally 

 manifested by the format ion of a layer of pigmented cells over 

 the operated half, the formation of the missing medullary fold 

 which grows from before backward or of the posterior parts of 

 both folds when we have an anterior half embryo. The com- 

 plete tadpoles resulting are in part active, in part weak, easily 



The internal changes that take place in tins post generation 

 as revealed by sections are the growth of a new eetoblast and 

 mesoblast into the previously imperfect cellular mass by a 

 process of successive rearrangement and differentiation of the 

 yolk cells, adding themselves to the edges of the advancing 

 layers in such wise as to form parts symmetrical with those in 

 the uninjured half of the embryo. The entoblast also is 

 formed from the free edge of the entoblast of the perfect half 

 by rearrangement of yolk cells and there is no invagination. 

 no gastrula nor blastula cavity formed in this newly forming ' 

 half of the embryo. The germ layers thus form by a sort of 

 regeneration from the interrupted surfaces of the old germ 

 layers, but as the material comes from the new half of the 



