238 THE MALE REPRODUCTIVE SYSTEM 



inward toward the lumen. A reduction of the chromosome number 

 from diploid to haploid is effected, and then a differential process 

 takes place as a result of which sperm are produced. The phases of 

 development up to and including reduction of the chromosome 

 number constitute spermatocytogenesis, and the following stage of 

 differentiation of the more or less spherical cells with haploid chromo- 

 some number into spermatozoa is called spermiogenesis. Not all 

 the tubules or all portions of any one tubule are actively producing 

 sperm at any one time. In an active tubule, there are rounded 

 cuboidal spermatogonial cells resting on the basement membrane 

 and undergoing mitosis. As each divides, one daughter cell is 

 pushed toward the lumen and in turn it divides so that one of 

 the daughter cells is again pushed further toward the lumen. 

 The number of mitoses, or spermatogonial divisions, appears to 

 show some variation in different animals, but the cells resulting 

 from the last mitosis of this early proliferation period are known 

 as primary spermatocytes, and they enter a resting phase during 

 which they increase in size. Later they undergo a maturation 

 process involving a reduction division and a following regular 

 mitotic division which produces the spermatids, small spherical 

 cells with a haploid chromosome number. 



After spermatids are formed they associate themselves closely 

 with the tapering distal portions of the Sertoli cells. As a result 

 of a transformation process, these small spherical cells become fully 

 developed spermatozoa, with head, middle-piece, and tail. The 

 head is composed of the condensed nuclear matter of the haploid 

 number of chromosomes; the middle-piece immediately behind the 

 head contains some of the cytoplasm and the centrioles; and the 

 tail is a long flagellum-like process. IVIuch of the cytoplasm of the 

 spermatid is sloughed off during differentiation of the sperm and 

 discharged into the lumen where it may join the sperm in their 

 passage toward the efferent ducts or be resorbed in part by the 

 Sertoli cells. During the process of spermiogenesis the head 

 remains buried in the Sertoli cell and the tail when formed extends 

 into the lumen of the tubule. When fully formed the sperm become 

 free of the Sertoli cells and pass into the lumen, but are themselves 

 as yet quite inactive. The sperm show some variations with regard 

 to size and shape, but the parts are fundamentally very similar. 



If a particular section of a seminiferous tubule shows activity. 

 a number of cells appear to be in the same stage of development. 

 If spermatids are present, a nutnber in the same stage ap])ear; 

 similarl.N- only s])ennatocytes in the same ])liase, or only spermato- 



