24 TEXT-BOOK OF EMBRYOLOGY. 



The spermatogonia multiply by ordinary mitosis. During the period of 

 sexual maturity of the individual, the spermatogonia are constantly proliferat- 

 ing, while at the same time, some of the spermatogonia, ceasing to proliferate, 

 enter upon a period of growth in size (Fig. 17). When the limit of this growth 

 is reached, these cells are known as primary spermatocytes. These large cells 

 ■ have distinct nuclear networks. As the primary spermatocyte prepares for 

 division, the spireme thread probably segments into one-half the usual number 

 of pieces. As already noted, this results in Ascaris in the formation of tetrads. 

 The process is not so clear, however, in higher forms, but in all cases, whether 

 there is tetrad formation or not, the behavior of the chromatin is such that 

 each daughter cell — secondary spermatocyte — receives one-half the usual num- 

 ber of chromatin segments.* Without any return of the nucleus to the resting 

 state, each secondary spermatocyte divides into two cells which are known as 

 spermatids, each of which also receives one-half the usual number of chromatin 

 segments, or chromosomes. Thus each primary spermatocyte gives rise to 

 four spermatids, each of which contains one-half the somatic number of 

 chromosomes. A careful study and comparison of the stages represented in Fig. 

 15, A, B, C and D will assist in understanding the processes of spermatogenesis. 



After the last spermatocyte division and the resulting formation of the 

 spermatid, the nucleus of the latter acquires a membrane and intranuclear net- 

 work, thus passing into the resting condition. Without further division the 

 spermatid now becomes transformed into a spermatozoon. This is accomplished 

 by rearrangement and modification of its component structures (Fig. 16). 

 The centrosome either divides completely, forming two centrosomes, or partially, 

 forming a dumb-bell-shaped body between the nucleus and the surface of the 

 cell. The nucleus passes to one end of the cell and becomes oval in shape. 

 Its chromatin becomes very compact and is finally lost in the homogeneous 

 chromatin mass which forms the greater part of the head of the spermatozoon. 

 Both centrosomes apparently take part in the formation of the middle piece. 

 The one lying nearer the center becomes disk-shaped and attaches itself to the 

 posterior surface of the head. The more peripheral centrosome also becomes 

 disk-shaped and from the side directed away from the head a long delicate 

 thread grows out — the axial filament. The central portion of the outer cen- 

 trosome next becomes detached and in Mammals forms a knob-like thickening — ■ 

 end knob — at the central end of the axial filament. In Amphibians this part of 

 the outer centrosome appears to pass forward and to attach itself to the inner 

 centrosome. In both cases the rest of the outer centrosome in the shape of a 

 ring passes to the posterior limit of the cytoplasm. As the two parts of the 

 posterior centrosome separate, the cytoplasm between them becomes reduced 

 in amount, at the same time giving rise to a delicate spiral thread — the spiral 



*According to Bonnet, the reduced number of chromosomes first appears in the spermatids. 



