lUXUCLEATE CELLS IN TISSUE CULTURES. 83 



The nucleus now became elongated and a refractive mass appeared in the 

 lowermost pole — apparently another nucleolus; at the same time the central nucle- 

 olus became a single mass, and was somewhat longer than before (29). Next, a 

 shallow notch formed in one side, and the nucleus became shorter and thicker, its 

 nucleoli undergoing minor changes (30 and 31). At the end of two hours the nucleus 

 again elongated and a deep notch appeared, indistinctly marked out on one side (32). 

 This seemed to become shallower in 33, but the presence of the centrosi)here pre- 

 vented this portion of the nuclear membrane from being well defined. 



The next change was the formation of another notch on the opposite side, both 

 notches forming what seemed like a zone of constriction about the nucleus. A 

 refractive mass stretched across the equator of the nucleus between these notches 

 (34). This is apparently a strand of mitochondria rather than a nucleolus, for, in 

 the next drawing (35), 2| hours after the observation began, this strand is situated 

 between two apparently separate nuclear portions, the nucleus having divided 

 directly. In no fixed and stained cell has a nucleolus been seen to occupy this posi- 

 tion; on the other hand, mitochondria have frequently been seen between these 

 nuclear parts, as in figure S. There was here no evidence of the formation of either 

 a spireme or an amphiaster, and thus Wilson's (1900) criterion for amitosis was 

 fulfilled. It may also be noted that the centrosphere did not divide and the nuclear 

 membrane remained intact. 



The final division apparently took place very rapidly, since the actual separation 

 was completed in the 15-minute interval between 34 and 35. This rapidity of the 

 end process of nuclear cleavage accounts for the infrecjuency of such terminal con- 

 stricting forms as figures 6 and 8, and makes the relatively small number of these 

 later transitional forms adequate to account for the number of binucleate cells which 

 originate therefrom. The cell was allowed to remain on the microscope stage all 

 night, but unfortunately wandered away and was lost, so the subsequent changes 

 could not he followed. The drawings were made from direct observation, but not 

 with the aid of the camera lucida. Mitochondria and centrospheres are partially 

 diagrammatic. This process, though traced with difficulty, and though somewhat 

 obscure, seems to follow the classic descrij^tions of amitotic division of the nucleus, 

 viz, elongation with equatorial constriction, forming a somewhat dumb-bell shaped 

 figure, and final se]iaration of the two nuclear portions. 



A similar elongated nucleus in a connective-tissue cell was followed for 6| hours, 

 and did not divide, but finally degenerated; in the meantime it underwent various 

 changes in shape and was rounded when last observed. Th(^ changes in nucleoli 

 were similar to those in figun^s 24 to 35. 



Thus it appears that a nucleus in a condition of elongation and constriction 

 may remain undivided for a long time and may even return to the rounded form 

 without dividing at all. In cases, however, where the constriction has passed a 

 critical point, as apparently was the case in the nucleus represented in figure 34, 

 the process of division i)r()ceeds rapidly. 



The study of fixed preparations, too, throws some light on nuclear amitosis, for 

 ill these one frefiu(>ntly finds nuclei evidently undergoing direct division. Such 



