540 BULLETIN OF THE 



As Auerbach first pointed out, the new nuclei in division arise in the 

 handle of the "dumb-bell." The early observers who make the centre 

 of the aster the seat of the forming nucleus are unquestionably in error 

 even if it must ultimately be granted that the " areal corpuscle " at the 

 centre of the aster forms subsequently an element in the new nuclear 

 structure. This corpuscle certainly cannot be looked upon as the begin- 

 ning of the new nucleus, which at an early stage lies at a comparatively 

 great distance from it. It has been shown by the observations of Biitschli 

 and Strasburger, and still more satisfactorily by those of 0. Hertwig, 

 that the new nuclei arise directly from the lateral zones of fibre thicken- 

 ings, which, in turn, owe their existence to a rearrangement of nuclear 

 substance. My observations serve to confirm this for Limax. 



How the metamorphosis of the lateral zones is effected has not been 

 so definitely established. Biitschli has claimed that the new nucleus 

 begins by the formation of a very small, clear, fluid-filled space around 

 the dark granules of each zone, and that the granules become the micleoli 

 of the new nucleus. Certain features of the metamorphosis in Limax 

 seem to favor this view. A comparison of Fig. 90, where the first evi- 

 dences of a segmentation furrow are visible, with Fig. 93, where the 

 cleavage is only about half completed, shows that the changes in the 

 nascent nucleus must be at this period very rapid. The existence of a 

 large number of nucleoli in the second case is perhaps indicative of a 

 direct genetic connection between the '' thickenings " and the nucleoli, 

 rather than a more radical metamorphosis out of which the nucleoli have 

 arisen as new formations ; and yet, in any event, time enough has elapsed 

 for a considerable increase of the nuclear mass, as the size of the new 

 nuclei clearly shows. In later segmentation stages, especially during 

 the second cleavage, I have more satisfactorily observed the early condi- 

 tion of the nucleus, — most distinctly after treatment with osmic acid 

 and Beale's carmine. When of about the size of the lateral zone in Fig. 90, 

 it appears as a small uniformly stained homogeneous body. The pro- 

 nuclei (Fig. 70'') are also sometimes encountered in a homogeneous con- 

 dition. But the rarity with which these stages are found lead me to 

 think they are of exceedingly brief duration. 



It is generally conceded that the nucleus is composed of at least two 

 substances, which present properties different from each other and from 

 the protoplasm of the cell. They have been designated by R. Hertwig 

 as " Kernsubstanz " and " Kernsaft." 0. Hertwig has explained the for- 

 mation of the new nucleus as due to a process the reverse of that which 

 takes place at the formation of the rods composing the middle zone of 



