140 THE THEORIES OF EVOLUTION 



minants of the left half of the organism, the other all 

 the determinants of the right half; one may contain 

 the determinants of the ectoderm and of all the organs 

 which will develop out of it, the other the determinants 

 of the endoderm with all its possible derivations. 



At the next cleavage, the blastomere containing the 

 determinants of all the ectodermic organs will be di- 

 vided up into two or more daughter-cells, one of which 

 will contain the determinants of all tegumina, the 

 other the determinants of the nervous system and so 

 forth. Ontogenesis is based upon the very fact that 

 every cleavage results in dissimilar blastomeres and 

 that these differences, becoming more and more no- 

 ticeable along the same line, produce in the end inde- 

 pendent structures due to purely internal causes. 



Releasing more and more of its determinants, as 

 organs and tissues become differentiated, the germ 

 plasm becomes less complex, more uniform and, when 

 tissues are entirely differentiated, transforms itself 

 into the idioplasm which is found in every cell and 

 which only contains determinants of one cell and of 

 parts thereof. Thereupon the determinants break up 

 into biophors which cross the nuclear membrane and 

 scatter themselves through the whole cytoplasm thus 

 imparting to the cell its distinctive character. When 

 the cell is not definitely differentiated but is destined 

 in the course of its development to transform itself 

 into another, some of its determinants are active (those 



