The Nebenkeiu iu the speimatogeueiis of Pulmonata — Helix and Aiion. 431 



Nebenkcrn, to which the cytoplasmic radiations which surround it 

 seem to converge. They are the centrosomes. In spermatocytes with 

 fine nuclear network I have not been able to see them, but they 

 soon become visible as the chromosomes become differentiated, and 

 are of considerable size before the beginning of mitosis. Hence we 

 must conclude that the Nebenkern is the attractionsphere of the 

 spermatocyte. The rodlets, "Nebenkernschleifen", are not independent 

 elements but merely optical sections of favorably lying thickened 

 portions of the much crumpled and folded outermost layer (ümhül- 

 lungsmembran, Sphärenhülle). To speak of them as present iu con- 

 stant number is at least hazardous, for the methods which give the 

 appearance of separate segments, do not enable us to observe them 

 from every direction. 



Quite recently Meves ^) has described and figured similar struc- 

 tures in Sdlamandra. The similarity of my Figs. 4 and 5 to his 

 fig. 39, and of my Fig. 21 to his fig. 42, tab. 3 is very striking. 



If we seek for an explanation of the occurrence of such a com- 

 plicated attractionsphere in resting spermatocytes, we must apparently 

 associate it with the long resting (i. e. not dividing) stage of the 

 growing period. Especially noteworthy in this connection are Meves' 

 pregnant remarks on the relative development of the attractionsphere 

 in spermatogonia which are dividing rapidly and those dividing more 

 slowly (1. c. p. 14). And p. 15: "Ebenso entstehen, wie wir sehen 

 werden, deutlich abgesetzte Sphären dann, wenn die kleinen Spermato- 

 gonien in das Ruhestadium der Spermatocyten übergehen." 



III. First Spermatocyte Mitosis. 



When the chromosomes, already well formed rings, begin to lose 

 their ragged appearance and become smooth, the centrosomes which 

 have shown a steady increase in size, separate slightly from each 

 other and approach the nuclear membrane (Figs. 5 and 7). They 

 then move rapidly apart and the next stage figured shows them al- 

 ready separated by a considerable arc of nuclear membrane (Figs. 8 

 and 12). In Fig. 8 the nucleus is no longer spherical but ovoid, its 

 long axis lying at right angles to the line joining the centrosomes. 

 This deformity is even better marked in Fig. 9 representing a some- 



1) Meves, F., Ueber die Entwicklung der männlichen Geschlechts- 

 zellen von Sal. maculosa, in: Arcli. mikr. Anat., V. 48, tab. 3, fig. 39, 

 42, 45, 46). 



