ESSENTIAL FEATURES OF SEXUAL PROCESS 



1207 



chromosomes in its nucleus. The same number is found in types so far 

 apart as the ox, the guinea-pig, and the onion. In the mouse, the sala- 

 mander, and the lily the number is twenty-four. Other types, such as the 

 crustacean Artemia, are said to have as many as 168 chromosomes, while 

 in Ascaris the cells only contain two or four chromosomes. All these changes, 

 which are included under the term mitosis or karyokinesis, seem to be 

 adapted to ensuring an equal qualitative as well as quantitative distribution 

 of the nuclear chromatin 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 



FIG. 556. Three stages of heterotype mitosis in spermatocyte of triton. (MooBE.) 

 a, germinal condition of chromosomes ; b, gemini arranged in quadrate loops or 

 tetrads ; c, separation of tetrads into the duplex chromosomes of the daughter 

 nuclei. 



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 number of 

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

 characteristic of the species, and since these chromosomes 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 forma- 

 tion 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 ' hetero- 

 type ' mitosis, or ' meiosis ' (Fig. 556). We may take as an example the 

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



