236 THE (iERM-PLASM 



This actually occurs before the germ-cells unite in the process 

 of 'reducing division' of the nuclear matter of the germ-cells. 

 This fact may probably be taken as indicating the correctness 

 at any rate of the fundamental idea on which the theory of the 

 germ-plasm is based, viz., that the hereditary substance is com- 

 posed of ids. These parts of this substance, the existence of 

 which I formerly concluded from purely theoretical considera- 

 tions, and which I have called • ancestral germ-plasms,' must 

 exist in reality. I venture to make this assertion with all the 

 more assurance, owing to the fact that at the time when I postu- 

 lated the 'reducing division' merely on theoretical grounds, the 

 existence of such a process could not be gleaned from recorded 

 observations even in the case of the female germ-cells of animals, 

 in w^hich it can be observed comparatively easily, quite apart from 

 that of the male cells of animals, or of the germ-cells of both 

 sexes in plants. 



We now know that this reduction of the number of ids by 

 one half is of general occurrence, and is effected by means of the 

 nuclear divisions which accompany cell-division. The divisions 

 which result in the formation of the. polar bodies perform the 

 function of the 'reducing divisions' as regards the ovum, and 

 the final divisions of the sperm mother-cells have this function 

 in the case of the spermatozoa. In both cases the reducing 

 division does not consist in the idants becoming split longitudi- 

 nally, and in their resulting halves being distributed equally 

 amongst the two daughter-nuclei as in ordinary nuclear division, 

 but in one half of the entire number of rods passing into one 

 daughter-nucleus, and the other half into the other. The process 

 is somewhat more complicated than would appear from this 

 statement, and it will be discussed more fully later on ; but the 

 final result is the same. 



The following considerations may perhaps help to explain 

 why the constant doubling of the germ-plasm could only be 

 prevented by this method of removing entire nuclear rods, and 

 will at the same time indicate what are the primary causes of 

 the changes in the stiaicture of the germ-plasm caused by 

 amphimixis. 



As already remarked, the nuclear rods must, before the intro- 

 duction of the process of amphimixis into the organic world, 

 have consisted of a number of identical ids, each corresponding 

 exactly to the individuality of the organism in question. These 



