IV. i INITIAL STRUCTURE OF THE GERM 



161 



Anterior half-embryos occurring when the second furrow by 

 an ( anachronism ' appears first were also observed, but posterior 

 never. 



Quarter and three-quarter embryos were obtained by killing 

 three or one of the first four cells, and Hemiblastulae superiores 

 by killing the four yolk-cells in the eight-celled stage. 



The self-differentiation of the Idioplasson is thus demonstrated. 

 Subsequently, however, the reserve Idioplasson comes into play, 

 and the missing half is formed by a peculiar process, to which 



A 



FIG. 77. A and B. Normal Frog embryos with medullary folds (m), 

 open (A) and closed (B). C. Hemiembryo dexter with almost complete 

 post-generation of the ectoderm ; , yolk-plug. D. The same, older, but 

 with less post-generation. E. Hemiembryo anterior (?) with beginning 

 post-generation. (From Korschelt and Heider, after Roux.) 



Roux has given the name of ' post-generation ' to distinguish it 

 from regeneration, in which lost parts are formed out of already 

 differentiated tissue. 



The injured blastomere first degenerates, is then reorganized, 

 and finally post-generated. 



In the first of these phases the cytoplasm and yolk are seen to 

 be vacuolated, while the injured but not completely killed nucleus 

 has given rise to small and large nuclei of normal structure 



JENKINSON M 



