; CHAPTER III. 

 MATURATION. 



It was stated in the preceding chapter that the essential condition for the 

 production of a new individual, in practically all the animal kingdom and with- 

 out exception among the Vertebrates, 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 chromosomes unless the latter 

 were reduced to one-half their normal number. Such 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 (Fig. 9) this epithelium consists of two kinds of cells: (i) the support- 

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

 ment. Of the latter the basal layer consists of small round or oval cells which 

 are known as spermatogonia. Internal to these are the larger spermalocytes 

 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 chromo- 

 somes, 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 (Fig. 9). 



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

 higher animals, such as mammals, has not been definitely shown on account 

 of the extreme minuteness of the cells and the difficulty of obtaining suitable 

 material. 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 (Fig. 10) and some of 

 the insects (Fig. 1 7) show the later stages with remarkable clearness. There 

 is no reason to suppose that the maturation processes of the mammalian germ 

 cells differ essentially from those of lower forms. 



The spermatogonia divide by ordinary mitosis, each daughter cell receiving 

 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 



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