82 JUNUCLICATE CKLL.S IN TIS.SUK CULTURES. 



Considering finally (d), it seems probable that these twin nuclei arise through 

 direct equal l)inary fission of the nucleus without division of the cyt()i)lasm. The 

 evidence upon which this assumption rests is, first, the inadequacy of other explana- 

 tory hypotheses; and, second, the observation in living cells of a process which is 

 apparently direct nuclear division, and the occurrence in fixed preparations of tissue 

 cultures of what must be regarded as transitional forms between single and d()ubl(> 

 nuclei. 



It is true, as Harman (1913) remarks (p. 219), that "the fact that two nuclei 

 lie in contact is no evidence that they have arisen by amitotic division," and in 

 the material which she studied, viz, early cleavage stages of Tcenia teniceformis and 

 Moniezia, she imd()ul)tedly presents convincing evidence that nucl(M which have 

 arisen by mitosis may lie quite close to one another within the same cell. This, 

 however, is a case of delayed cleavage, for she states (p. 215) : 



"In cleavage, nuclear division takes place very much in advance of cytoplasmic 

 division. In the early divisions it is the exception and not the rule to find even a constric- 

 tion in the cytoplasm. This gives rise to a syncytial condition. This syncytium persists 

 until very late cleavage." 



This is quite a different condition from that obtaining in the cells of tissue 

 cultures. Then, too, many of her nuclei contain spiremes. Her contention in 

 no way counts against the view that the double nucl(>i of tissue cultures are of 

 amitotic origin. 



Observations on direct nuclear fission will now be recorded, first to be described 

 being the process as it was seen to occm- in the living cell. As has been pointed out, 

 it is impossible to tell from inspection of the living culture which of the thousands 

 of mononucleate cells will divide directly, and so to follow the process of nuclear 

 amitosis in the living cell it is necessary to select a cell which shows some indication 

 of beginning direct division, i. e., by elongation and constriction. Figures 24 and 

 25 appear to be typical of the early stages of direct division of the nucleus. 



Many attempts to trace the changes in such a cell were made, with, however, 

 onlj' partial success, for in almost every case the nucleus lost its constriction and 

 became rounded again, or the cell degenerated. However, one case was found where 

 what appeared to be direct division of the nucleus occurred during observation. 

 The various phases are shown in the series of figures 24 to 35, which were drawn at 

 15-minute intervals from a single cell growing in a culture from a 5-day chick heart 

 in Locke solution with extract from chick embryo. The culture was of 57 hours' 

 duration. A cell was first selected which contained an elongated nucleus with a 

 marked notch in one side. In this notch the centrosphere was situated, and conse- 

 quently this side was .somewhat indistinctly outlined (24). Instead of dividing, the 

 cell straightened out, almost losing the indentation (25). It contained two nucleoli, 

 one situated in the uj^permost pole, and the other, which was paired, about the 

 equator. The nucleus next became rounded (26 and 27) and, after one hour's obser- 

 vation, its outUne was almost circular (28) . In the latter figure there appeared tf) be 

 only a single paired nucleolus. 



