198 GENERAL PHYSIOLOGY 



the other half toward the other pole, so that by this second divi- 

 sion each nucleus obtains only one-half as many chromatin-fibres 

 as in a normal division (Fig. 79). 



Fig. 79. — Reducing division in tlie origin of tke sperm-cell from the sperm mother-cell of the 

 thread-worm of the horse. (After O. Hertwig.) 



These comprise the various forms of cell-division which have 

 become known thus far. The only element common to them all 

 is the transfer of both nuclear substance and protoplasm to the 

 daughter-cells. 



8. Fertilisation 



The act of fertilisation is intimately associated with that pro- 

 found mystery with which mankind is wont to invest its most 

 sacred feelings. The biologist recognises that fact that the un- 

 conscious aim of normal sexual love, one of the most powerful 

 factors that control organic life, is the microscopic act of fertilisa- 

 tion of the female egg-cell by the male sperm-cell. At first sight 

 it might seem strange that so powerful motives, as are those of 

 love in human life, . culminate in so tiny a phenomenon, which 

 cannot be perceived >by the naked eye ; but when it is borne in 

 mind what the result of this act is, what an endless chain of com- 

 plex processes and changes associated with the development of the 

 new organism from the egg is caused by fertilisation, and what is 

 the end-result of this long series of developmental processes — 

 namely, the highly complex animal, man, with the immeasurable 

 richness of his life — then this fact loses its strangeness, and we 

 come to attribute to the tiny act of fertilisation an extraordinary 

 significance, which it contains in potentia. It is no wonder, there- 

 fore, that since early times physicians and men of science have 

 made sexual reproduction the subject of deep research. Yet it was 

 not till after Leeuwenhoek had constructed the microscope that 

 his pupil, Ludwig van Hammen, discovered the sperm-cells, which 

 because of their active intrinsic movements were called " sperm- 

 animalcules " or " spermatozoa." And only the unlooked-for 

 perfection of the microscope in the present time has made possible 

 the brilliant work of Biitschli, Fol, Hertwig, van Beneden, Boveri, 



