IV. i INITIAL STRUCTURE OF THE GERM 159 



self-differentiating substance, the Idioplasson, in the nucleus, a 

 view adopted and elaborated by Weismann, while the facts of 

 regeneration necessitated the assumption of a reserve Idioplasson 

 endowed with the potentialities of the whole, which is not quali- 

 tatively divided during development, but handed on intact to 

 some or all of the tissues of the body. 



The hypothesis then assumes the existence in the nucleus of the 

 fertilized ovum of as many separate units as there are separately 

 inheritable characters in the embryo, arranged there according 

 to a plan which conforms to the structural arrangement of the 

 parts which they represent, the separation of these units by con- 

 tinued ' unlike ' nuclear division, and their distribution to the 

 cytoplasm, where they determine the formation of the structures 

 to which they are beforehand assigned. 



For Roux the theory was no mere speculation, but the 

 result of observation and experiment. The natural occurrence 

 of half or partial monsters, the correspondence of defects in the 

 embryo to injuries to the egg, and the coincidence of the first 

 furrow with the sagittal plane in a large percentage (80 %) of, 

 unfortunately, only a small number of eggs, suggested the self- 

 differentiation of predetermined parts, while the last-mentioned 

 observation led directly to the experiment in which one of the 

 first two blastomeres of the Frog's egg was killed and a half- 

 embryo produced from the survivor. 



By means of a hot needle Roux succeeded in at least partially 

 destroying one of the first two blastomeres. The other con- 

 tinued to segment, and passed through the ordinary stages ; 

 first the Hemimorula, with animal and vegetative cells and a 

 segmentation cavity (though the last was sometimes absent), 

 then by continued cell-division the Hemiblastula, followed by 

 the Hemigastrula and Hemiembryo. In all cases the living 

 tissues formed an exact half either a right half or a left half 

 of a normal embryo, and ended abruptly at the (sagittal) plane 

 of separation of the living and the dead. The segmentation 

 cavity, however, was observed to extend in some cases across 

 this plane, while in others it was confined wholly within the 

 living cells. 



In the Hemigastrula the greatest extent of the archenteron 



