REPRODUCTION 39 



endowed with a power of self-determination which lies 

 in the specific structure of its nucleus, and its course of 

 development is only in a minor degree capable of modi- 

 fication through the relation of the cell to its fellows." 



Weismann conceives that the chromatin (idioplasm) 

 of the germ-cell exists in the form of minute particles 

 which combine to form aggregates, and these again unite 

 to form compound groups and so on until finally we have 

 the chromosomes. He calls the smallest groups deter- 

 minants, the next larger groups ids (chromatin granules), 

 these in turn combining and forming idants or chro- 

 mosomes. Weismann in explanation of his theory says: 

 " Ontogeny depends on a gradual process of disintegra- 

 tion of the id of germ-plasm, which splits into smaller 

 and smaller groups of determinants in the development 

 of each individual. Finally . . . only one kind of 

 determinant remains in each cell, viz. that which has to 

 control that particular cell or group of cells. In this 

 cell it breaks up into its constituent biophores and gives 

 the cell its inherited specific character." 



In developing this theory it is necessary to assume a 

 very stable condition of the germ-plasm. For example, 

 one must assume that some portion of the original germ- 

 plasm is passed on to the germ nucleus unchanged in 

 structure from generation to generation. 



The theories of both Roux and Weismann are in part 

 only based upon demonstrated phenomena occurring in 

 the cell. Some of the most fundamental and far-reaching 

 postulates of these theories are highly speculative and 

 cannot be demonstrated by any known method of re- 

 search. It must be admitted, however, that Weismann's 

 theory of the germ-plasm with some modifications comes 

 nearer to an adequate explanation of the changes which 



