UIXrCLEATE CELLS IX TISSUE CULTURES. 9'.i 



in the living cell, from resting twin nucleus through mitosis to two separate and 

 distinct daughter cells, would seem to be proof absolute that these figures 22 and 23 

 (which represent a phase of this process) are prophases of combining double nuclei. 



The mere contact of two spireme-bearing nuclei (such as appear in figure 23), 

 is of itself no evidence that they will combine, but when we bring to bear upon the 

 interpretation of such a figure the evidence derived from a series such as that shown 

 in i)late iv, in which a nuclear formation, like that of figure 23, represents a stage, 

 it seems obvious that these nuclear parts are undergoing fusion to form a single 

 plate of chromosomes. Harman (1913) shows several figures of such nuclei in early 

 cleavages of Taenia teniceformis and Moniezia (her fig. c, plate 8), but here the sepa- 

 rate nuclei have arisen by mitosis, according to her observations, and cleavage, 

 which is delayed, will eventually separate the blastomeres. The nuclear memln-anes 

 are here quite intact, and show no evidence of beginning dissolution. 



It may be objected that the condition of spireme is no indication that mitosis 

 is beginning. To this the reply may be made that in the cells of living tissue-cultures 

 a nucleus showing a spireme of this kind, no companion cell in the same condition 

 being present, always represented the prophase of mitosis. 



Since mitosis occurs in binucleate cells ni vitro, it might be assumed that it 

 would also occur in such cells in vivo, and indeed this is the case, for ]\Iaximow 

 (1908) has found figures in fixed preparations from the mesenchyme of embryo 

 rabbits which strongly resemble those just described. In his figure 7 (p. 93) the 

 spireme is forming in a dumb-bell-shaped nucleus, and in his figure 8 the nuclear 

 fragments in which the spireme is found are quite separate. Maximow believes 

 that his pictures represent the jirophase of normal mitosis occurring in amitotic 

 nuclei; this belief is supported by my observations upon the living cell shown in 

 plate IV. In his figure 8 he finds the centriole-pair situated between the two coils 

 of the spireme — a position corresponding to that characteristic for it in the amitotic 

 nucleus, viz, in the cleft. In my figure 23, which is slightly later, the centrosome 

 has shifted its position to the pole. He states that his results resemble the findings 

 of Karpow (1904) for urodele amphibia; this latter author described a process of 

 nuclear amitotic division, with subsequent formation of a spireme from the frag- 

 ments (which may be two or more in number), with fusion to form one "mutter- 

 stern." It may theTefore be concluded, from the finding of such double spiremes in 

 eml)ryonic tissue, that this process of mitosis in binucleate cells occurs in lun-mal devel- 

 opment. It is thus to be found in differentiating as well as non-differentiating cells. 



I regret that I have seen no other living examples of combination of the parts 

 of a double nucleus during mitosis, but the process is so rare that its observation 

 thus is largely a matter of chance hitting upon a favorable cell. Mitosis occurs 

 rather infrequently in the mononucleate cell, and when it is considered that the 

 proportion of binucleate cells to total cells is very low (1 to 11 1) the remoteness of 

 the possibility of finding a binucleate cell which will divide by mitosis may he 

 realized. It is only in those cultures showing abundance of both binucleate cells and 

 mitotic figures that there is any hope of finding sucli compound mitoses. 



To ascertain the relative frequency with which mitosis occurred among the 

 l)iiiuclcaf(' cells, as compared with the mononucleate, a study, by careful counting 



