THE INDIVIDUAL AND THE SPECIES 183 



rods then split longitudinally, and one-half of each 

 moves towards one pole of the nucleus, the other half 

 moving towards the other pole. Various other modi- 

 fications of the cell and nucleus occur concomitantly 

 with these changes, but the essential thing that happens 

 seems to be the halving of all the structures of the cell, 

 and this is the simplest explanation of the phenomena 

 of mitotic cell division. Two daughter-cells are then 

 formed by the division of the mother-cell, and each of 

 these daughter-cells receives one-half of each of the 

 chromatin granules that were contained in the mother- 

 cell. 



The chromosomes, or " Idants," are seen to consist 

 of discrete granules, and these are (generally) the 

 bodies known as the " Ids." The id cannot be resolved 

 by the microscope into any smaller structures : it lies 

 on the limits of aided vision ; but the hypothesis 

 assumes that it is composed of parts called " Deter- 

 minants," and the determinants are further supposed 

 to consist of " Biophors." The biophors are the 

 ultimate organic units or elements, and they are of 

 the same order of magnitude as chemical molecules. 

 We must suppose them to be more complex than a 

 protein molecule, and the latter contains many 

 hundreds (at least) of chemical atoms. Now it is 

 possible to calculate the number of atoms contained 

 in a particle of the same size as the id : such a calcula- 

 tion may be made by different methods, all of them 

 yielding concordant results. This calculated number 

 of atoms may be less than that which we must 

 suppose to be present in the biophors, of which the 

 hypothetical id is composed ! 1 



1 " But," says Weismann, referring to an objection of this nature, " it 

 should rather be asked whether the size of the atoms and molecules is a 

 fact, and not rather the very questionable result of an uncertain method of 

 investigation." 



