310 LECTUKES ON BIOLOGY 



kinesis of the hereditary substance was found by many in Boveri's 

 observations of a ' chromatin-reduction ' of the nuclei during 

 the development of the ovum of Ascaris megaloceplialus, for in 

 these nematodes it is possible to distinguish already at the stage of 

 the two-cell segmentation the cells of the ' germ-path ' from the 

 somatic cells : only the stem-mother of the future reproductive 

 cells retains her nuclear rods unchanged and with her full com- 

 plement of chromatin, but in the body-cells both ends of the 

 chromosomes are regularly rejected and perish, only the central 

 sections being transmitted to subsequent cell-generations. We 

 cannot pursue this process in its details and must content our 

 selves with the admission that a ' germ-path ' can here actually 

 be distinguished from the body-cells, and that the body-cells 

 have suffered a diminution in their ' hereditary substance.' But 

 it seems to me that this fact speaks rather against than for 

 the occurrence of heterokinesis, for if mitosis can already effect a 

 qualitative separation of the ' complement of determinants ' and 

 transmit with this hypothetical precision to the body-cells only 

 those ' primary constituents ' that belong to them, there would 

 seem to be no need for the subsequent chromatin-reduction. 

 That a qualitative diminution of ' the hereditary substance ' does 

 take place subsequent to the division is a sign that simple nuclear 

 division is unable to effect it, but can only always lead to homoeo- 

 kinesis. It can certainly not be denied that direct observation 

 supplies no proof whatsoever of the occurrence of heterokinesis. 

 On the other hand, many arguments may be adduced against 

 this assumption. 



Among modern investigators it is in particular 0. Hertwig 

 who submitted the doctrine of inequivalent nuclear division to 

 sharp criticism. His statement sums up the case so tersely that 

 I will quote his own words : ' What purpose in the life of the 

 cell is served by the division in which nuclear segmentation 

 plays the leading part? Assuredly only the purpose of propa- 

 gation and reproduction and these are the means employed by 

 Nature for effecting the preservation of the organism as species. 

 The organism which is perishable as a single individual is 

 multiplied in its characteristics by reproduction and preserved as 

 species. 



* Of plants and animals we know from experience that each 



