Chap 3 LIVING MATTER AND CELLS 43 



its sharpness. The daughter cells are now complete growing cells in the inter- 

 phase stage. 



The time required for the complete process of cell division varies greatly 

 with the kind of cell and the surrounding conditions, especially temperature. 

 A cell of a salamander's heart observed living in tissue culture completed 

 the process in two hours. The process may be much quicker. 



Results of Mitosis. Two cells are formed that are identical with one 

 another in respect to every gene and every chromosome. This is accomplished 

 first by the doubling of the genes in the chromosomes, and then by the sepa- 

 ration of the chromosomes and their inclusion in the new nuclei. The re- 

 mainder of the cell may or may not be equally divided. In the growth of a 

 multicellular animal, whether hydra or man, mitosis is repeated thousands to 

 billions of times, and each time hereditary qualities originally received from 

 the parents and contained in the first cell are distributed equally to new cells. 



In amitosis the nucleus simply constricts into an hourglass shape and then 

 separates into two parts without forming chromosomes. This is a very rare 

 arrangement which occurs only under unusual conditions, especially in de- 

 generating cells. 



Reproduction of Sex Cells — Mitosis and Meiosis. Body cells reproduce 

 exclusively by mitosis. Germ or sex cells reproduce by mitosis and meiosis. 



The reproduction of sex or germ cells in males and females includes an in- 

 crease in numbers from a few original germ cells, a reduction to half their 

 number of chromosomes, i.e., from the diploid to the haploid number, and 

 changes in the shape and size of the cells (Fig. 3.10). The all-important genes 

 inherited from the parents of the individual and present in the chromosomes 

 of his or her original germ cells are distributed so that each gamete (egg and 

 sperm cell) has an inheritance from its ancestors, even remote ones. The 

 process in the male is spermatogenesis, the history of the sperm cell from its 

 earliest stage to maturity, and in the female, oogenesis, the history of the egg 

 cell. There are differences in size and numbers of the mature sex cells in the 

 male and female, but the changes in their nuclei are essentially similar. 



Spermatogenesis. The original primordial germ cells in the male divide 

 repeatedly by mitosis, gradually producing great numbers of extremely 

 minute, nearly spherical cells called spermatogonia. These have the diploid 

 (or body) number of chromosomes; half of them were in the male cell or 

 sperm and half in the female cell or egg when fertilization occurred. 



Suppose, for example, that a primordial germ cell has six chromosomes, 

 three derived from each parent (Fig. 3.10). Such cells divide mitotically, 

 producing several generations of cells called spermatogonia, each one of 

 which contains six chromosomes. A change then occurs beginning with the 

 maturation or meiotic divisions. First the cells become relatively larger and 

 are called primary spermatocytes. In the prophase of the first meiotic division 



