MATURATION 15 



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 in 

 the spermatogonia. Each spermatid now becomes transformed into a 

 mature spermatozoon (Fig. 10). 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 



Spermatozoa 



Sp'c.I (prophase) ... 

 Sustentacular < 



Connective-tissue^ 

 wall 



Spermatid *';' 



Sp'c. II (telo phase).. 

 Sp'c.II (metaphase)^ 



II (tehphase) 



imary spermalocyte 

 Accessory chromo- 

 some (f) 

 >'c. I (melaphase) 



<'c. I (prophase) 

 Spermatogonium 



Sp'g. (anaphase) 



FIG. 9. Stages in the spermatogenesis of man arranged in a composite to represent a portion 

 of a seminiferous tubule sectioned transversely. X 900. 



become the axial filament, and the cytoplasm forms the sheaths of the 

 neck and tail. The spiral filament of the connecting piece is derived from 

 the cytoplasmic mitochondria. 



The way in which the number of chromosomes is reduced may be 

 seen in the spermatogenesis of Ascaris (Fig. n). Four chromosomes are 

 typical for Ascaris megalocephala bivalens and each spermatogone contains 

 this number. In the early prophase of the primary spermatocyte there 

 appears a spireme thread consisting of four parallel rows of granules (5). 

 This thread breaks in two and forms two quadruple structures known as 

 tetrads (D-F) ; each is equivalent to two original chromosomes, paired side 



