GERM CELLS AND THEIR FORMATION 127 



heavy reticulum, and then into a dense mass in which no visible 

 structure remains; finally it acquires the form of the head of 

 the mature spermatozoon. The spermatosphere meanwhile is 

 largely converted into the acrosome or perforatorium, at the 

 tip of the elongated nucleus; a smaller portion is transformed 

 into a very delicate envelope covering a part of the head, in 

 many cases scarcely distinguishable on account of its thinness. 

 At the other end of the cell a fine flagellum begins to grow out 

 in connection with the peripheral centrosome, either from the 

 substance of the centrosome itself, or from the cytoplasm under 

 its influence. Then the two centrosomes move farther in 

 toward the nucleus. In the simpler cases the distal centro- 

 some now divides into anterior (toward the head) and posterior 

 portions. The posterior part grows out peripherally into a 

 rapidly elongating fiber which becomes the axial filament of the 

 flagellum, while at its base it becomes ring-like; then through 

 this ring or annulus the axial filament grows in, finally connect- 

 ing with the anterior portion of the centrosome. The anterior 

 portion itself remains in the middle piece as a posterior centro- 

 some of the spermatozoon. The proximal or anterior centro- 

 some partly disappears, and partly is converted into that part 

 of the middle piece which connects with the head (the neck). 



The cytoplasmic part of the spermatid seems to be largely 

 consumed in providing the energy for this transformation, in 

 addition to that drawn from the nurse or Sertoli cells. But the 

 cytoplasmic membranes of the middle piece and tail, including 

 the undulatory membrane when this is present, are derived 

 directly from the cytoplasm of the spermatid. The mitochondria 

 of the spermatid seem to be transformed into the spiral layer 

 of the middle piece. In many instances, particularly among 

 the Mammals, the larger part of the cytoplasm remains for a 

 time connected with the middle piece of the developing sper- 

 matozoon and then is cast off, and finally degenerates without 

 taking any further part in the structural formation of the 

 functional sperm cell. The chief structural correspondences 

 between spermatid and spermatozoon are shown in the accom- 

 panying table. 



