24 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



selves into a reticulum and daughter nuclei are formed. 2. The cytoplasm di- 

 vides in a plane perpendicular to the axis of the mitotic spindle. Two complete 

 daughter cells have thus arisen from the mother cell. 



The complicated processes of mitosis, by which cell division is brought about 

 normally, seem to serve the purpose of accurately dividing the chromatic sub- 

 stance of the nucleus in such a way that the chromatin of each daughter cell may 

 be the same qualitatively and quantitatively. 



This is important if we assume that the chromatic particles of the chromosomes bear the 

 hereditary qualities of the cell. The number of chromosomes is constant in the sexual cells 

 of a given species. The number for the human cell is in doubt.. It has been given as 16, 24, 

 and 32. According to Winiwarter's recent work, the number of chromosomes in each immature 

 ovum or oocyte is 48, in each spermatogone 47. Wiemann {Amer. Jour. Anat., vol. 14, p. 461) 

 finds the number of chromosomes in various human somatic cells varies from 34 to 38. In 

 species of Ascaris megalocephala, a parasitic worm, but two or four chromosomes are found 

 and in their cells the processes of mitosis are most easily observed. 



We have seen that reproduction in mammals is dependent upon the union of 

 male and female germ cells. The union of two germinal nuclei (pronuclei) 

 would necessarily double the number of chromosomes in the fertilized ovum and 

 also the number of hereditary qualities which their particles are supposed to bear. 

 This multiplication of hereditary qualities is prevented by the processes of matu- 

 ration which take place in both the ovum and spermatozoon. 



MATURATION 



Maturation may be defined as a process of cell-division during which the 

 number of chromosomes in the germ cells is reduced to one-half the number 

 characteristic for the species. 



The spermatozoa take their origin in the germinal epithelium of the testis. 

 Their development, or spermatogenesis, may be studied in the testis of the rat; 

 their maturation stages in the testis tubes of Ascaris. Two types of cells may be 

 recognized in the germinal epithelium of the seminiferous tubules, the sustentacu- 

 lar cells (of Sertoli), and the male germ cells or spermatogonia (Fig. 9). The 

 spermatogonia divide, one daughter cell forming what is known as a primary 

 spermatocyte. The other daughter cell persists as a spermatogone and, by con- 

 tinued division during the sexual life of the individual, gives rise to other primary 

 spermatocytes. The primary spermatocytes correspond to the ova before matu- 

 ration. Each contains the number of chromosomes typical for the male of the 

 species. The process of maturation consists in two cell divisions of the primary 



