26 



THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



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, the spermatids possessing 

 just half as many chromosomes as the spermatogonia. Each spermatid now be- 



Fig. ii. — Reduction of chromosomes in spermatogenesis in Ascaris megalocephala (bivalens) 

 {Brauer, Wilson). A-G, successive stages in the division of the primary spermatocyte. The original 

 reticulum undergoes a very early division of the chromatin granules which then form a doubly split 

 spireme (B) . This becomes shorter (C) and then breaks in two to form two tetrads (D) in profile, (E, in 

 end). F, G, H, first division to form two secondary spermatocytes, each receiving two dyads. /, 

 secondary spermatocyte. /, K, the same dividing. L, two resulting spermatids, each containing two 

 single chromosomes. 



comes transformed into a mature spermatozoon (Fig. 10), the nucleus forming the 

 larger part of the head, the centrosome dividing and lying in the neck or middle 

 piece. The posterior centrosome is prolonged to form the axial filament, and the 

 cytoplasm forms the sheaths of the middle piece and tail. 



