84 lilxrCLKATIC CELLS IX TISSUE CULTURES. 



forms are to be regarded as transitional stages between the mononucleate and 

 binucleate cell. Figure 2 shows a nucleus which has undergone elongation and 

 equatorial constriction, so that there is an indentation on either side. The nucleolus 

 appears to be dividing also; this condition of the nucleolus is, however, not constant. 

 Figure 3 shows a cell in which constriction is somewhat farther advanced; here the 

 nucleoli have apparently divided, two being seen in each nuclear portion, in these 

 cells the method of jireparation does not show cytoplasmic details. 



Figures 6 and S show nuclei in which direct division is almost com])lete, the 

 nuclear parts being held together only by the fniest filament. Similar nuclear iigures 

 were found by IMaximow (1908) in embryonic rabbit tissue, as shown in his figure 1, 

 and the upper two nuclei in his figure 10. In figure 8 the nucleus has divided 

 unequally, and the larger ]iortion contains two nucleoli, while the smaller h;is but 

 one. In figure 6 only one nuclear portion contains a nucleolus, and this is single. 

 In both cells the unchanged centrosphere is situated in its characteristic position 

 between the two nuclear portions, while the mitochondria radiate out from this bodj^ 

 and a strand of mitochondria passes over the bridge connecting the nuclear parts. 



There is nothing in the api)earance of these nuclei to suggest the late telojihase 

 of an intranuclear mitosis, such as those shown by Cary (1909) and referred to by 

 Richards (1911, p. 158). The clearness characteristic of the cells of tissue cultures 

 prevents confusion of nuclear amitosis with the late telophase of mitosis, such as 

 has been shown by Richards (1909) to be jjossible in the cells of Tcenia. 



In figure 9 nuclear sejiaration has been completed, the two portions being ([uite 

 free from one another. These are of about ecjual size and appearance, and each 

 contains two nucleoli. Mitochondria and centrosomes occupy their typical posi- 

 tions; the former are short rods, this being a cell from heart membrane. 



In figure 7 the separate nuclear parts have come together and their surfaces 

 are just touching. Mitochondria ha\'e been forced out, but the centrosphere is 

 characteristically opposite the area of contact of the nuclear portions. Figure 4 

 shows a somewhat similar binucleate cell, from a Zenker and Mallor>- preparation. 



It would appear that the nucleus may sometimes divide by a gradually deep- 

 ening cleavage from one side, which finally cuts it into two pieces. This may be 

 regarded as an asymmetrical type of constriction. Figure 5 may be taken as repre- 

 sentative of the beginning of this process and figure 6 the end. The centrosphere is 

 found typically in the notch, as has many times been recorded in amitotically 

 dividing nuclei, as by Maximow (1908). In the rare exceptions to this rule the 

 centrosphere may have been originally situated in the notch and subseciuently 

 have left it. No evidence of separation of the centriole-pair during nuclear ami- 

 tosis has been found. 



Richards (1911. p. 156) finds constricted and indented nuclei in his material 

 only in cases of imperfect fixation. \Miatever may l)e said as to the nuclear distor- 

 tion brought about by many fixatives, this is not an explanation of such figures as 

 6 and 8 seen in tissue cultures, for here osmic-acid vapor was used as a fixative and 

 this does not change the nuclear outline, as may be i)roved by ob.serving a living 

 nucleus and the same nucleus after fixation (Lewis and Lewis, 1915). Then, too, 

 only a small proportion of nuclei appear thus, whereas if the ap})earances were to 



