34 THE GERM-PLASM 



it must be so. and that tlie units of the idioplasm on which the 

 nature of the latter depends are far too numerous, and therefore 

 far too small, to be visible. 



If therefore the two halves into which the chromatin rods are 

 split in karyokinesis look exactly alike, and even if the divided 

 portions of the granules (microsomes), of which the rods often 

 visibly consist, resemble each other exactly, there is still no 

 reason why they should not be different in their nature ; in some 

 cases one. and in others another occur. 



We shall consequently in this connection have to assume two 

 kinds of nuclear division which are externally indistinguishable 

 from one another, in one of which the two daughter-nuclei con- 

 tain similar idioplasm, while in the other they contain different 

 kinds of idioplasm. These kinds of division we may speak of 

 as hof/iceokinesis and heterokinesis, that is, as a division into 

 parts similar or dissimilar to each other with regard to the hered- 

 itary tendencies they contain ("erbgleich' and • erbungleich ') . 

 The former must depend on a perfectly uniform distribution of 

 the primary constituents in the two halves of the rods, and will 

 consequently have been preceded by a duplication in the process 

 of growth ; in the latter this growth will be connected with a 

 heterogeneous grouping of these constituents. 



Although we cannot ascertain anything directly about the 

 forces which cause this splitting of the chromatin rods, it may at 

 any rate be asserted that they must be contained within the sub- 

 stance of the latter, and be connected with the actual develop- 

 ment of the qualities of the idioplasm : for otherwise it could 

 not be understood how the qualities, which are changed during 

 the division of the nucleus, become separated sharply from one 

 another and arranged in the two daughter-nuclei. And yet this 

 must be the case if different cells with different kinds of idioplasm 

 can all arise from one mother-cell, which is an undoubted fact. 



It appears to me, therefore, that the regular ontogenetic 

 changes of the idioplasm, as they begin with the division of 

 the egg-cell and cease with the natural death of the organism, 

 depend on purely internal causes, which lie in the physical nature 

 of the idioplasm. In obedience to these, a division of the nucleus 

 accompanies each qualitative change in the idioplasm, in which 

 process the different qualities are distributed between the *.wo 

 resulting halves of the chromatin rods. I shall speak of the 

 different kinds of idioplasm arising in this way as the ontogenef- 

 stages of the idioplasm, or shorth-, tlie onto-idic stages. 



