322 GRIFFIN. 



the meganucleus is pulled out into a thread at the point of division ap- 

 pear to be expressions of the internal tensions existing during this period, 

 and, to a certain extent at least, are evidence supporting the theor}- of 

 Butschli quoted above, that the elongated shape of the nucleus is the 

 result of and is maintained by cell-tensions. The contractility of the 

 nuclear membrane and of the intranuclear reticulum are factors which 

 seem sufficient to account for the concentration of the meganucleus, but 

 they can not cause its elongation. During the process of concentration 

 the karyolymph has been almost completely expelled from the nucleus. 

 Absorption of karyolynifih during expansion increases the volume of the 

 meganucleus, but can not of itself direct expansion in particular direc- 

 tions. This must be accomplished by tractive force exerted upon the 

 nucleus by the cytoplasm, of which we have visible evidence in the 

 temporary- stretching of the reticular mesh. Figures 30 and 31, Plate 

 YII, are drawings of Euplotes immediately after fission. In figure 31, 

 the anterior end of the nucleus still remains drawn out into a point which 

 ends Just inside the pellicle. This point, as well as other ii-regularities 

 of the anterior end of the nucleus, would have been lost very quickly, for 

 the nucleus grows into its ordinary form soon after division. 



DIVISION OF THE MICRONUCLEUS. 



The micronucleus divides much more quickly than the meganucleus, 

 commencing after the reconstruction phase of the latter has started, and 

 being completed some time before that phase has ended. The chromatin 

 of the resting micronucleus exists in the form of a reticulmn, which is 

 only visible after thorough extraction of the stain. (Plate lY, figures 1, 

 2, and 3.) Soon after reconstruction of the meganucleus has been 

 entered upon, the micronucleus increases in size to about double its or- 

 dinary dimensions. The enlargement appears to be due to an increase in 

 the fluid contents, for no change in the chromatin can be seen. 



Eearrangement of the chromatin commences when the reconstruction 

 bands of the meganucleus have proceeded about one-quarter of their 

 distance. The chromatin meshes first become elongated in the direction 

 of the anterior and posterior poles of the micronucleus; next they may 

 be found as threads reaching from end to end of the nucleus with a 

 slightly spiral twist. (Plate V, figure 10.) It is evident that the 

 chromatin threads are increa.sing in thickness and staining power. At 

 the same time the micronucleus swells still more. The spindle is formed 

 by-the elongation of these threads of chromatin which stretch from pole 

 to pole. (Plate IT, figure 5.) I have not been able to distinguish 

 purely linin fibers at any stage of the ordinary division mitosis, for all 

 spindle fibers appear to contain chromatin. Appearances in the mitosis 

 of conjugation lead me to believe that during ordinary mitoses the linin 

 fibrils are completely covered by or otherwise inseparably joined to the 

 chromatin. 



