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nucleus — chromatin — breaks up into a number of thread-like 

 bodies, known as chromosomes ; and the cells of every species of 

 plant or animal has a definitely fixed number of these bodies. 

 This number is always even in the cells of the sporophyte and 

 in the somatic cells of animals. In the cells of the gametophyte 

 and gametes of plants and animals it is frequently uneven, 

 where the number may be as low as one. When a cell is about 

 to divide each chromosome splits longitudinally, the cell thus 

 for a brief period containing double the normal number ; when 

 division ensues the doubled number of chromosomes is equally 

 divided between the d ughter cells, reducing the number once 

 again to the normal. This brief and obviously incomplete 

 description of mitosis applies to soma — body — cells only. Germ- 

 cells — eggs and sperms — are multiplied in the same way, 

 i.e., by mitosis, but with this essential difference, excepting in 

 parthenogenesis they contain but half the normal number of 

 chromosomes, for at a certain period of their history the 

 number of chromosomes normal to the soma cells is halved, 

 the products — daughter cells — germ-cells — thus containing only 

 half the original number ; this is the reduction division, to 

 which reference will be made later. Cells possessing this 

 reduced number are generally regarded as incapable of further 

 division, and eggs and sperms alike degenerate and die, unless 

 one of each are brought together, thus providing the full 

 complement of chromosomes in the zygote. 



The chromatic material of the nucleus is now generally 

 regarded as the material basis of heredity, the substance 

 bearing the characteristics of the race, and at the same time 

 apparently carrying also the peculiarities of the parents. If, 

 however, the results of the experiments of Boveri and Delage 

 must be accepted as conclusive, the importance of the nucleus 

 and its contents have been somewhat over-rated ; for they 

 have shown that denucleated eggs of the sea-urchin can be 

 fertilized, and that normal larvae are the result. Further, that 

 if spermatozoa of a different sub-species be used, the 

 characteristics of the larvae are those of the male. This 

 must suffice as to the nature of the germ-cells. 



