14 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



cephala univalens, a round worm parasitic in the intestine of the horse. The largest number 

 known is found in the brine shrimp, Artemia, where 168 have been counted. 



The number for the human cell is in doubt. Guyer (1910) and Montgomery (1912) 

 found 22 in the spermatogonia of negroes, and Guyer (1913) reported considerably larger 

 numbers (count not given) for white spermatogonia. According to Winiwarter's recent 

 work on whites (Arch, de Biol., T. 27, 1912), the number of chromosomes in each immature 

 ovum or oocyte is 48, in each spermatogone 47. Wieman (1913) found the most frequent 

 number in various white somatic cells to be 34, but recently (Amex Jour. Anat., vol. 21, 1917) 

 he asserts that the number in both negro and white spermatogonia is 24, thereby agreeing 

 with Duesberg (1906). 



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 quahties 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 man or of 

 the rat; their maturation stages in the tubular testis of Ascaris. Two t3^es of 

 cells may be recognized in the germinal epithelium of the seminiferous tubules, 

 the sustentacular cells (of Sertoh) , and the male germ cells or spermatogonia (Fig. 

 10). The spermatogonia divide, one daughter cell forming what is known as a 

 primary spermatocyte. The other daughter cell persists as a spermatogone, and, 

 by continued division during the sexual life of the individual, gives rise to other 

 primary spermatocytes. The primary spermatocytes correspond to the ova 

 before maturation. 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 spermatocytes, each producing first, two secondary spermatocytes, 

 and these in turn four cells known as spermatids. During these cell divisions the 

 number of chromosomes is reduced to half the original number, the spermatids 

 possessing just half as many chromosomes as the spermatogonia. Each spermatid 

 now becomes transformed into a mature spermatozoon (Fig. 11). The nucleus 

 forms the larger part of the head; the centrosome divides, the resulting moieties 

 passing to the extremities of the neck. The posterior centrosome is prolonged to 



