ROUX'S EXPERIMENTS ON THE EGG 129 



material is present at the beginning of development has 

 been proved at any rate for embryonic organs of many 

 species. Thus pre-formation (in the above sense) in the 

 earliest stages, at least of embryonic organs, is confirmed, 

 although it is not necessarily accompanied by pre- 

 localization. 



Turning to the details of Roux's researches upon the 

 frog, briefly mentioned on p. 1 28, the results he claimed 

 to produce by destroying one or both of the anterior or 

 posterior pair of the first four cells (blastomeres) formed 

 by the second cleavage are not now accepted. As 

 regards the injury to one of the first two cells (blasto- 

 meres) and the development of a half-embryo from the 

 other, Roux stated that after a time the material 

 of the injured cell is ' reorganized ', and the missing 

 half of the embryo restored by ' post-generation '. 

 The reorganization is, according to Roux, of complex 

 origin, in part due to formation of cells from the injured 

 part itself, but chiefly to the influence of the uninjured 

 part. When converted into cells by reorganization, post- 

 generation begins. 'A few hours or a night is sometimes 

 sufficient to change a hemi-embryo into a whole embryo.' 

 ' The pieces of the old nucleus . . . may take a part in 

 the formation of the new cells ; wandering cells migrate 

 from the yolk mass of the old half into the new, and the 

 cells of the formed germ-layers may be pushed over to 

 the other side.' This brief recapitulation of Roux's 

 account is quoted from T. H. Morgan, who has himself 

 shown that a half-embryo is formed by the uninjured 

 cell when it and the injured cell retain their original 

 position, but that the missing part is regenerated from 

 the uninjured half or from the still living material of the 

 injured cell. Morgan also proved that if the two first 

 ceils do not retain their original position e.g. if the egg 

 be turned upside down the uninjured cell forms not 

 a half but a whole embryo, as in Hertwig's experiments. 

 Furthermore, Schulze and Wetzel have proved that the 

 uninjured egg, if kept upside down in the two-celled 

 stage, develops into a double monster, one from each 

 cell (blastomere). We are thus driven to the conclusion 



