'Cell Division in Relation 

 to Reproduction 



SOONER or later each kind of cell reaches 

 a size at which it must either stop growing or 

 divide. If it is to be division, the cell begins 

 to undergo a drastic reorganization of both 

 its nuclear and cytoplasmic structure. In- 

 deed, a considerable part of the metabolic 

 activity of the cell during its life between 

 divisions appears to be directed toward the 

 synthesis of new materials in preparation for 

 the next division. The cell is thus ready to 

 replicate itself, and all the important and 

 dramatic events of cell division can begin to 

 proceed without delay and with great preci- 

 sion. 



Biologists are generally agreed that new 

 cells arise solely by division from pre-existing 

 parent cells, at least under conditions as they 

 exist today. Consequently cell division pro- 

 vides the underlying basis for all forms oi re- 

 production — sexual as well as asexual (p. 

 51) — in every kind of plant and animal. Cell 

 division also provides a basic mechanism for 

 the transmission of hereditary qualities from 

 cell generation to cell generation, and from 



40 



generation to generation of the whole organ- 

 ism. 



In man and in multicellular animals gen- 

 erally, all the cells of the body arise by divi- 

 sion from a single embryonic cell, the fertil- 

 ized egg. This fertilized egg, formed by the 

 fusion of two cells, an egg and a sperm, like- 

 wise arises by division from the cells of the 

 parent organisms. The same situation also 

 holds true lor most plants. However, the cells 

 of some plants are formed by the multiplica- 

 tion of another kind of cell, called a spore, 

 which differs somewhat from an egg cell 

 (Chap. 12). 



In both plants and animals a great major- 

 ity of the cells are produced in the same way, 

 by a type of cell division called mitosis, 

 which was described brieflv in Chapter 2. 

 But it is important to realize that certain 

 reproductive cells — especially the eggs and 

 sperm in animals, and the spores in plants — 

 are produced by a somewhat different tvpe of 

 cell division called meiosis. These two tvpes 

 of division, although somewhat modified in 



