BINUCLEATE CELLS l.\ TLSSUE CULTURES. 89 



The division of the nucleolus thus seems to have nothing to do with the separa- 

 tion of the nucleus; indeed, after the nucleolus has divided, the nucleus may not 

 divide at all. It may, however, have to do with the size of the nuclear portions; 

 where these latter are equal they each contain one or two nucleoli, of about equal 

 size, whereas where they are unequal one portion — usually the smaller — may not 

 contain a nucleolus. 



That direct division of the nucleus may take jilace without preliminary fission 

 of the karyosome in tissues developing normalh' is CA'ident from the statement of 

 Wilson (1900, p. 115): "In many cases, however, no i)rehminary fission of the 

 nucleolus occurs; and Remak's scheme must, therefore, be regarded as one of the 

 rarest forms of cell division." It is interesting to note that Schultz finds evidences 

 of such a simj^le form of direct di\ision in the nuclei of cells of the same tumor in 

 which he finds "promitosis." 



Summing up, then, the process of direct nuclear fission, it is probable that vari- 

 ous factors are involved. Elongation of the nucleus is undoubtedly sometimes 

 followed by its cleavage, and, since it is always present in nuclear amitosis, it maybe 

 regarded probably as an essential in this. The activity of centrosphere and mito- 

 chondria must also be considered as a factor in equal, as well as unequal, nuclear 

 fission, and this activity is apparently made effective by nuclear elongation. 



Fission of the nucleolus, while possibly concerned with the relative size of the 

 nuclear parts, is not necessarilj' associated with the initiation or carrying out of 

 nuclear cleavage. 



Inasmuch as binucleate cells, and constricted nuclei which must be regarded 

 as their precursors, are found in apjiarently normal embryonic tissue, they can hardly 

 be considered as abnormal or as evidence of a reversion to a more primitive type of 

 cell division; furthermore, their healthy condition is manifest from their capacity 

 to divide by mitosis, as will be shown hereafter. Thus it is reasonable to suppose 

 that the factors operative in nuclear division in tissue cultures are those which 

 function in embryonic cells in vivo. 



Since these binucleate cells seem to represent the first step on the road to 

 certain giant cells it may be concluded that the latter are the result of a repetition 

 of the same processes which bring about the formation of the former. This view is 

 in accord with that of Lewis and Lewis (1915), p. 391, who state: "These giant cells 

 appear to be formed by an amitotic division of the nucleus without a coincident 

 division of the cytoplasm." 



FATE. 



The nucleus having divided directly, what becomes of it? Obviously the most 

 certain method of settling this (juestion is to select a living binucleate cell and watch 

 it constantly as it passes through its various changes. This course has been followed 

 with several cells, and the evidence at hand does not show that the cell as a whole 

 divides otherwise than by the regular process of mitosis; in the early stages of this 

 process there is a combination of the two nuclear portions to form a single mitotic 

 figure. 



Plate IV is a series of camera-lucida drawings representing successive stages in 

 the history of one of these twin nuclei, in a living connective-tissue cell, grown from 



