GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 165 



may, perhaps, be obtained by taking the spermatogenesis of annelids 

 and insects as a basis of comparison. In the insects (butterflies), 

 Biitschli showed, in 1871, that the tail is formed by an elongation 

 of the cell-body, into which extends the elongated Nebenkern, now 

 divided into two longitudinal halves (Fig. 82). Platner ('89), confirm- 

 ing this observation, further showed that the Nebenkern (in Pygcera) 

 consisted of two parts, stating that one ("large mitosome") gives rise 

 to the investment of the axial filament, the other (" smaH mitosome ") 

 to the middle-piece ; while a third still smaller body, described as a 

 " centrosome," passes to the apex. The later works of Henking('9i) 

 and Wilcox ('95, '96) render it nearly certain that Platner confused 

 the acrosome with the centrosome, the first-named observer finding in 

 Pyrrhocoris and the second in Cahptenus that Platner's " centrosome" 

 is derived from the Nebenkern, while Wilcox traced the centrosome 

 directly into the middle-piece. Paulmier, finally, has shown in Anasa 

 that the axial filament grows out from the centrosome, 1 proving that 

 such is the case by the highly interesting observation that in giant 

 spermatozoa, arising by the non-division of the primary or secondary 

 spermatocytes, either two or four centrosomes are present, each of 

 which gives rise to a single axial filament, though only one Nebenkern 

 is present (Fig. 82). (The bearing of this important fact on the 

 centrosome-question is indicated elsewhere.) These observations, 

 made on three widely different orders of insects, seem to leave no 

 doubt that in insects the centrosome lies in the middle-piece (i.e. at 

 the base of the nucleus), while both the acrosome and the inner tail- 

 envelopes are derived from the Nebenkern. The outer envelope of 

 the tail is derived from unmodified cytoplasm. 



In the earthworm the phenomena are slightly different, the middle- 

 piece arising from an idiozome or attraction-sphere (Calkins, '95), in 

 which lies the centrosome (Erlanger, '96), while the Nebenkern seems 

 to have no part in the formation of either acrosome or tail-envelopes. 2 



We turn now to the Amphibia, elasmobranchs, and mammals, in 

 which the same general result has been attained, though there is still 

 some divergence of opinion regarding the exact history of the centro- 

 some. Working on the basis laid by Flemming ('87, '88), Hermann 

 ('89) traced the middle-piece in the salamander to a " Nebenkb'rper," 

 which he believed to be not a Nebenkern but an attraction-sphere, 



1 Moore ('95) seems to have been the first actually to describe the outgrowth of the axial 

 filament from the centrosome, in the elasmobranchs. It has since been described by Meves 

 ('97, 2) and Hermann ('97) in the salamander, by Lenhossek ('97), Meves ('98, '99), and 

 Bardeleben ('97) in the rat, guinea-pig, and man; by Godlewski ('97) and Korff ('99) in 

 Helix, and by several others. 



2 Calkins's preparations, which I have carefully examined, seem to leave no doubt that the 

 middle-piece arises from a true attraction-sphere derived from the spindle-poles; but 

 Erlanger believes that the grarrnlar " centrodeutoplasm " also contributes to the sphere. 



