1350 



PHYSIOLOGY 



term mitosis or JcaryoJcinesis, seem to be adapted to insuring an equal 

 qualitative as well as quantitative distribution of the nuclear chroma- 

 tin among the daughter cells resulting from the division of any cell. 

 As we have seen in an earlier chapter, the nucleus, by its interaction 

 with the cytoplasm, determines the processes of assimilation and 

 growth of the whole cell. For the preservation of type in cell division 

 it is therefore essential that the nuclei of the daughter cells shall be 

 identical in all respects with the nucleus of the mother cell. 



Sexual reproduction involves the conjugation of two cells, with 

 union of their nuclei. If each of these nuclei consisted of the normal 





FIG. 554. Three stages of heterotype mitosis in spermatocyte of triton. 



(MOORE.) 



a, geminal condition of chromosomes ; &, geniini arranged in quadrate 

 loops or tetrads ; c, separation of tetrads into the duplex chromosomes of the 

 daughter nuclei. 



number of chromosomes, the fertilised egg-cell would contain double 

 the number characteristic of the species, and since these chromo- 

 somes would divide by splitting, the number of chromosomes in each 

 cell would be doubled with each generation. This doubling is obviated 

 by the fact that, in the formation of the germ-cells, the ovum and 

 spermatozoon nuclei undergo a special type of division, which leads 

 to the reduction of the chromosomes in the sexually mature cell to one 

 half of the number characteristic of the species. This mode of cell 

 division is often called ' division by reduction,' or ' heterotype ' mitosis, 

 or ' meiosis ' (Fig. 554). We may take as an example the development of 

 spermatozoa. The mother cells of the spermatozoa, the spermatocytes, 

 divide twice, giving rise to four daughter cells, the spermatids, each of 



