SPERMATOGENESIS OF THE PIG. 23 



centrosomes, from the outer one of which the axial filament grows 

 out and the centrosomes later move toward the nucleus. In the 

 pig the centrosomes are never peripherally located and division 

 of the spermatid centrosome does not take place until it emerges, 

 surrounded by a small clear sphere, from the main bulk of the 

 centrosphere which is in close contact with the nucleus. 



It can be seen from the drawings of the later stages that the 

 nucleus gradually elongates and flattens (Figs. 67-78). The 

 large chromatic mass within the nucleus disintegrates and the 

 particles become distributed at the periphery of the nuclear wall 

 (Fig. 78). The tail envelope becomes apparent as the cell 

 elongates and evidently develops from the cytoplasm (Figs. 74- 

 78). It later comes in contact with the axial filament and en- 

 velops about half of its entire length. The remaining extremely 

 thin portion of the tail is apparently the naked axial filament 

 (Figs. 76, 77, 78). 



When the cells reach the stage represented in Figs. 71-73 

 they attach themselves in bunches to the large cylindrical Sertoli 

 or nurse cells that often extend from the basement membrane a 

 long distance toward the lumen of the tubule (Figs. 2 and 63). 

 As the spermatozoa continue to develop a gradual decrease in the 

 volume of the cytoplasmic mass of the Sertoli cells is noticeable. 

 When the spermatozoa are apparently mature they abandon 

 the Sertoli cells which at this time are very much collapsed. 

 The spermatozoa after leaving the nurse cells do not pass directly 

 to the lumen of the tubule, but remain scattered for some time 

 among the masses of cytoplasm which were cast off by the same 

 developing cells a short time before (Figs. 3 and 67). This cast- 

 off material does not scatter about in the lumen, as one would 

 suspect, but forms a sort of loose layer next to the cells following 

 the maturing spermatozoa. The masses begin to disintegrate 

 soon after they are discarded by the developing spermatozoan, 

 and when stained with iron-haematoxylin numerous black bodies 

 make their appearance in this material (Fig. 3). The spermato- 

 zoa remain, with their bodies embedded in this layer, and tails 

 extending into the lumen, until the mass almost completely dis- 

 appears. Then they become free in lumen of the tubule and are 

 ready to make their way out of the testes. During this period 



