The Nebenkern in the spermatogenesis of Pulmonata — Helix and Alien. 433 



it is considerable. Such appearances seem to me quite incompatible 

 with the view, held by Lee, that the radiations (for his "entonnoirs 

 polaires" can be nothing else), are centred around pores in the 

 nuclear membrane. The discovery of centrosomes in the Nebenkern 

 prior to mitosis is corroborative evidence of the most indisputable 

 kind that the processes of cell division in the spermatogenesis of 

 Helix conform to the generally accepted type. 



IV. The Nebeiikern during the preceding Changes. 



When the centrosomes separate from each other the Nebenkern 

 undergoes important changes. It suifers disintegration iato a number 

 of fragments which separate into two groups, one around each centro- 

 some. Still preserving this arrangement they accompany the centro- 

 somes to the poles of the nucleus (Figs. 7, 12—15, 19, 22). I have 

 been unable to count the number of fragments in each group. In 

 late prophases a marked inequality in this respect is often noticeable 

 between the two radiations. In addition several lobules frequently 

 separate en masse, and pass to one or the other pole (Figs. 14 and 

 22 ; cf. Meves' figures of "Sphaerenbrocken", 1. c). In the amphiaster 

 the Nebenkern fragments lie so that they are seen from the edge 

 when the achromatic figure is seen in profile (Figs. 15, 23), from the 

 surface when looked at from the pole (Fig. 10 a). It is extremely 

 difficult to decide whether they lie between the radii or are connected 

 with them. During the metaphase I have only obtained fragmentary 

 evidence of their continued existence in Helix, but they are easily 

 demonstrable in telophases (Fig. 16). In Arion the series is quite 

 complete (Figs. 24 and 25). Here we can see how the Nebenkern 

 fragments lose their orderly arrangement with the involution of the 

 radial system and coalesce to form the Nebenkern of the spermato- 

 cytes of the second order. 



A similar series of figures (Figs. 18—20) is given for the second 

 ripening division of Helix which now ensues, the chromatin of the 

 daughter nuclei having in the meantime formed a rather coarse reti- 

 culum (Fig. 16 and 17). The amphiaster of the second ripening 

 division is sharply distinguished from that of the first, by the smaller 

 size of the cells, rather finer cytoplasmic reticulum and the loosely 

 apposed parallel daughter chromosomes (cf. Figs. 15 and 20). 



The number of ring chromosomes entering into the equatorial 

 plate of the first ripening division is twenty-four. The number of 

 pairs of sister-chromosomes in the second division is probably the 



