158 FOOT-NOTES TO EVOLUTION. 



no attempt has been made even to indicate the many- 

 variations in detail which occur in different animals and 

 plants. These are numerous, but do not affect the gen- 

 eral plan nor the fundamental goal, which is always and 

 invariably the same — viz., the equal, longitudinal divi- 

 sion of the chromosomes or the chromatin band of the 

 parent nucleus between the two daughter nuclei. 



Another method of cell division, formerly taken to 



be the universal one, has since been shown to occur 



solely as a stage in the degenerative 



,. !^^^^ changes of cells which are upon the 



division. ° .... • J • 



downward road to disintegration, and in 



which the power of multiplication is about at an end. 

 In this, the " direct " or " amitotic " form of cell divi- 

 sion, the cell nucleus is simply constricted into two por- 

 tions preceding the constriction of the cytoplasm. This 

 method stands in marked contrast to the elaborate 

 mechanism which insures the exact distribution of the 

 nuclear substance in the karyokinetic or indirect method. 



Every multicellular organism arises by a process of 

 division from a single cell, the fertilized germ or egg 

 cell, which in turn has been cut off from the cells of a 

 pre-existing individual. Out of the group of cells which 

 result from the continued division of the germ cell and 

 its descendants are differentiated the various tissues and 

 organs of the body through which the vital functions are 

 carried on. Those tissues and organs which perform 

 functions pertaining directly to the existence of the indi- 

 vidual have been termed " somatic," and 

 " Somatic " and ^^^-^^ constituent cells the " somatic " or 

 reproductive , , ,, • ,• • • ^u 



body cells, in contradistinction to the 

 tissues. ■' ' 



reproductive tissues or cells whose func- 

 tion concerns the continuance of the species. In some 

 forms these two groups of cells, the somatic and the 



