WEISMANN'S MAIN TENETS 211 



from a solitary spermatozoon and a solitary ovum can produce 

 on the average, as already indicated, 85,000,000,000 other 

 spermatozoa each resembling the primordial spermatozoon in 

 size and properties, it is clear that— leaving the body cells out 

 of account — the germ plasm contained in that primordial sper- 

 matozoon has multiplied itself several thousands of millions of 

 times ; or otherwise it has in the process of growth assimilated 

 vast quantities of other material to itself, and rearranged it so 

 that that new material has come to possess the same constitution, 

 and, with this, the same properties as itself. With growth there 

 is constant new formation of molecules forming the individual 

 cells. In other words, the germ plasm is not eternal ; it is 

 constantly being renewed. What are the nearest to being potenti- 

 ally eternal are the chemical and physical properties of the germ 

 plasm. And this very fact, that growth implies constant re- 

 arrangement and assimilation of constituent molecules, makes 

 the chemical composition " eternal " only so long as the assimil- 

 able material remains the same. To this I shall revert. Weis- 

 mann, it is true, admits that there is growth ; he does not, 

 however, realize all that this necessitates. 



He next demonstrates very clearly that this heritable germ 

 plasm is contained in the nuclei of the conjugating ovum 

 and spermatozoon, and regards as a proved proposition 

 that the nuclear chromatin is the hereditary substance. This 

 is present in the germ cells of every species in the form of a 

 definite number of chromosomes which, in cells destined for 

 fertilization, is reduced to one-half the number, so that the 

 single nucleus — the segmentation nucleus — of the fertilized ovum 

 contains the number of chromosomes characteristic of the cells 

 of the individual of any particular species. The hereditary 

 substance of the child is therefore formed half from the maternal, 

 half from the paternal substance, and as, at each succeeding 

 cell-division, each of the paternal and maternal chromosomes 

 doubles by dividing, each cell of the individual contains hereditary 

 material derived from both father and mother. 



This very fact of reduction, he points out, indicates that half 

 the chromosomes contain all the essential primary hereditary 

 constituents, or, otherwise, that it is not the chromatin as a whole 

 that conveys hereditary properties. The rather, this must be 

 made up of a series of constituents or ids, each of which is repre- 



