94 CYTOPLASMIC STRUCTURES [CH. 



young spermatid, just after the last division, the centrosome 

 is commonly just below the surface, and begins to grow out 

 a filament which projects outside the cell-protoplasm. In 

 some animals this outgrowth into a projecting filament may 

 begin at an earlier stage; in Moths, for example, it occurs 

 in the primary spermatocytes and the two centrosomes 

 each develop two filaments, which separate from one another 

 when the centrosomes divide for the second spermatocyte 

 division (PL XI, 2, 3). More commonly the filament is only 

 developed after the last cell division is completed; it grows 

 out rapidly, and at the same time the centrosome itself sinks 

 in to the neighbourhood of the nucleus, and sometimes 

 pushes in the nuclear membrane so as to appear as if en- 

 closed within the nucleus. At first the filament growing from 

 the centrosome is naked, but as the spermatid becomes 

 converted into the spermatozoon its cytoplasm grows out 

 along the filament and gives rise to the cytoplasmic sheath 

 or fin surrounding the axial filament of the tail. The tail 

 thus arises from a filament which grows out from the sper- 

 matid centrosome and becomes clothed with a thin sheath 

 of cytoplasm. The middle-piece at the same time originates 

 chiefly from the centrosome itself and the thickened base 

 of the axial filament, surrounded by part of the mito- 

 chondrial apparatus next to be described (PL X, a-g). 



The formation of the middle-piece and tail is not, how- 

 ever, a simple growing out or lengthening of the cytoplasm 

 of the spermatid around the axial filament produced from 

 the centrosome, but is accompanied by important processes 

 for the description of which it will be necessary to turn back 

 to the earlier stages of spermatogenesis. In the young 

 primary spermatocyte, when suitably stained, granules or 

 strands may often be seen in the cytoplasm, especially in the 

 neighbourhood of the centrosome. These bodies are the 



