GROWTH AND DIFFERENTIATION OF THE GERM-CELLS l6/ 



rise to the middle-piece. According to Meves, who has most thor- 

 oughly studied the entire formation of the spermatozoon, the history 

 of these parts is as follows : In .the young sp.ermatids the two centro- 

 somes lie quite at the periphery of the cell (Fig. 84), 1 and from the 

 outer one grows out the axial filament. The two centrosomes, leav- 

 ing the idiozome by which they are first surrounded, now pass 

 inwards toward the nucleus, the outer one meanwhile becoming trans- 

 formed into the ring mentioned above, while the axial filament passes 

 through it to become attached to the inner centrosome. The latter 

 pushes into the base of the nucleus and enlarges enormously to form 

 a cylindrical body constituting the main body of the middle-piece. 

 The ring meanwhile divides into two parts, the anterior of which 

 gives rise to a small, deeply staining body at the posterior end of the 

 middle-piece identical with the "end-knob." The other half of the 

 ring wanders out along the tail, finally lying at the limit between 

 the main part of the latter and the end-piece. The envelope of the 

 axial filament, here confined to that side opposite the marginal fin 

 (i.e. the "ventral " side of Czermak), is formed by an outgrowth of the 

 general cytoplasm along the axial filament. The fin and marginal 

 filament are believed by Meves, as I understand him, to be formed 

 from the axial filament ('97, 2, p. I2/). 2 The acrosome, finally, is 

 formed from the idiozome which wanders around the nucleus to its 

 anterior pole. McGregor's results on Amphiuma ('99) agree in their 

 broader features with those of Meves, but differ on two points, one of 

 which is of great importance. The acrosome here arises from only 

 a part of the sphere (idiozome), while a second smaller part passes to 

 the base of the nucleus and forms the main part of the middle-piece. 

 The inner centrosome passes into the middle-piece to persist as the end- 

 knob from which the axial filament passes out into the tail (Fig. 84). 

 The history of the sphere thus recalls the phenomena seen in the Ne- 

 benkern of the insect-spermatid ; though the posterior moiety does not 

 contribute to the tail-envelope, while the history of the inner centrosome 

 is somewhat like that observed in the mammals, as described beyond. 

 In the elasmobranchs Moore ('95), Hermann ('98), Suzuki ('98), and 

 Benda ('98) likewise traced the spermatid-centrosome into the middle- 

 piece (Fig. 85), and Moore first showed that from it the axial filament 

 grows out. 3 Moore derived, both middle-piece and acrosome from the 



1 Cf. their position in epithelial cells, p. 57. 



2 Hermann ('97) gives a somewhat different account of the process, believing that the 

 ring is derived from the mid-body of the last mitosis. Meves and McGregor have, however, 

 shown that the ring and mid-body coexist in the early spermatids (Fig. 84), which seems 

 decisive against Hermann's conclusion. 



3 Hermann Tncls also the ring observed in the salamander, and believes it to be the mid- 

 body. The middle-piece is regarded by him as a product of the spindle-remains, but on 

 both these points he is contradicted by Suzuki. 



