14 THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



Anaphase. At this stage the two groups of daughter chromosomes 

 separate and move up along the central spindle fibers, each toward one of 

 the two asters. Hence this is called the diaster stage (V, VI). At this 

 stage, the centrioles may each divide in preparation for the next division 

 of the daughter cells. 



Telo phase. i. The daughter chromosomes resolve themselves into a 

 reticulum and daughter nuclei are formed (VII, VIII). 



2. The cytoplasm divides in a plane perpendicular to the axis of the 

 mitotic spindle (VIII). Two complete daughter cells have thus arisen 

 from the mother cell. 



The number of chromosomes is constant in the cells of a given species. The smallest 

 number of chromosomes, two, occurs in Ascaris megalocephala 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) 

 counted 22 in the spermatogonia of negroes. For white spermatogonia, Guyer (1913) 

 reported considerably larger numbers (count not given) than he had formerly found in the 

 negro. This is suggestive in view of Winiwarter's (1912) apparently careful work on whites 

 which gave for the oocyte 48, for the spermatogone 47; although this enumeration needs 

 confirmation, it has been tentatively accepted by many. Wieman (1913) found the most 

 frequent number in various white somatic cells to be 34, but recently (1917) he asserts 

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

 Duesberg's (1906) count. 



MATURATION 



We have seen that reproduction in vertebrates follows upon the union 

 of male and female germ cells. Without special provision this union 

 would necessarily double the number of chromosomes at each generation. 

 Such progressive increase is prevented by the processes of maturation 

 which take place in both the ovum and spermatozoon. 



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. Its significance in the mechanism 

 of inheritance is discussed on p. 22. 



Spermatogenesis. The spermatozoa take their origin in the germinal 

 epithelium of the testis. Their general development, or Spermatogenesis, 

 may be studied in the testis of man or of the rat; the details of their 

 maturation stages in Ascaris or in insects. Two types of cells are recog- 

 nized in the germinal epithelium of the seminiferous tubules: the susten- 

 tacular 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 spermotocyte. The other daughter cell persists as a spermato- 

 gone, and, by continued division during the sexual life of the individual, 



