CHAPTER II. 

 MATURATION. 



It was stated in the preceding chapter that among the vertebrates the 

 essential condition for the production of a new individual was the union of 

 two sexually different cells. Since the number of chromosomes is constant 

 for all the cells of a species, such a union would cause a doubling of chromo- 

 somes unless the latter were reduced to one-half of their normal number. 

 Such a reduction actually takes place, .and forms the essential part of the 

 maturation processes of the germ cells. 



SPERMATOGENESIS MATURATION OF THE SPERM. 



The spermatozoa arise from the germinal epithelium of the testis. In 

 the mammal this epithelium consists of two kinds of cells: (i) the supporting 

 cells (of Sertoli) and (2) the spermatogenic cells in various stages of develop- 

 ment (Fig. 6). Of the latter the basal layer consists of small round or oval 

 cells which are known as spermatogonia. Internal to these are the larger 

 spermatocytes having large vesicular nuclei with densely staining chromatin. 

 Between these and the lumen of the seminiferous tubule are several layers of 

 small round or oval cells, the spermatids. The spermatids have the reduced 

 number of chromosomes, and by direct transformation give rise to the mature 

 spermatozoa which may either lie free in the lumen of the tubule or have their 

 heads embedded in the supporting cells. 



The way in which the maturation or reduction divisions take place in the 

 higher animals, such as mammals, is difficult to demonstrate on account of 

 the small size of the cells. The following account is based on data obtained 

 from the study of lower forms (amphibia, fishes, insects, Ascaris) whose 

 maturation processes have been demonstrated with great accuracy. Ascaris 

 and some of the insects show the later stages with remarkable clearness. 

 It is reasonable to suppose that the maturation processes of the mamma- 

 lian germ cells agree essentially with those of lower forms. 



The spermatogonia divide by ordinary mitosis, each daughter cell receiv- 

 ing the full or diploid number of chromosomes. After several generations ^ 

 some of the spermatogonia pass through a period of growth and are then 

 known as primary spermatocytes. During this period important changes 

 take place in the nucleus. The chromatin granules become concentrated 



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