Units of Replication 31 



number of chromosomes. The latter appears to be the more com- 

 mon; it is referred to as endopolyploidy. In some organisms (e.g., 

 insects) each tissue has its own characteristic degree of polyploiciv. 

 In the salivary glands of water striders (Gerris) the highly special- 

 ized cells may be 2,048-ploid. Tissues of other organisms contain a 

 population of cells of varying degrees of ploidy (usually with a 

 norm at a level above diploidy, e.g., at the tetraploid or octaploid 

 level). This is the situation in the human Hver, for example, or in 

 tissues in the roots and stems of the flowering plants. 



The significance of the phenomenon of endopolyploidy is not well 

 understood. Certainly it correlates with secretory activities of the 

 cells, and it may play a role in development as well as in differentia- 

 tion. It should be emphasized, however, that no qualitative changes 

 in the genetic material have been demonstrated. Germ-line cells in 

 animals and cells producing spores in plants usually do not become 

 endopolyploid. Reproductive cells therefore retain the zygotic or 

 gametic number of chromosomes, while somatic cells may experi- 

 ence successive increases. If vegetative reproduction in plants takes 

 place by means of budding or suckering from somatic tissue, off- 

 spring with increased chromosome number may arise. Such situa- 

 tions are discussed in Chap. 9. 



CELL DIVISION: M E I S I S 



In addition to these mechanisms of preserving the existing chromo- 

 some number or of increasing it, organisms obviously must have a 

 mechanism for reducing it. Mechanisms for reducing high endopoly- 

 ploid numbers are poorly understood, but they have been reported in 

 insects (Ciilex) and plants { Allium). The great majority of organ- 

 isms share the mechanism for reducing the zygotic number of chro- 

 mosomes to the gametic number; this mechanism is called meiosis 

 (Fig. 2.4). Meiosis occurs in tissues that have not undergone endo- 

 polyploidy, such as germ-line cells in animals or sporogenous tissue 

 in plants. Again, as with mitosis, its outlines are the same in all 

 organisms, although in animals it results in gametes and in most 

 plants it results in spores. A cell about to undergo meiosis may be 

 called a meiocyte. The results of meiosis are nearly always four 

 daughter cells with half the number of chromosomes of the meiocyte. 

 In the formation of eggs in animals and in the production of mega- 

 spores in plants, three of these may be much smaller and e\entually 

 disappear. 



Meiosis achieves these results by two cell diN'isions but only one 

 division of the chromosomes. The following description refers to the 



