THE MITOTIC CYCLE 



lymph-sac of a frog a fragment of elder pith, which became infiltrated 

 by wandering cells. It was subsequently removed and mounted in a 

 moist chamber on the stage of the microscope; the leucocytes then 

 wandered out and could be observed in such preparations for a period 

 of four or five days. Arnold deserves a place in the history of tissue 

 culture, and his experiments would be well worth repeating with the 

 use of the phase microscope. In his paper, Arnold gives several series 

 of drawings of leucocytes in amitosis. Each begins when the process is 

 already well advanced, and both the nucleus and cell have elongated 

 and are constricting in the middle (Figure 60). The later stages from 

 this point to the separation of the two cells occupied 30-45 minutes. 

 It is rare for the amitotic division of a nucleus to be followed by the 

 cleavage of the cell; another instance where this seems to occur is 

 provided by the erythrocytes of Necturus, according to Charipper and 



)0(Xt^ 



Figure 60 Stages in the amitotic division of a living Frog leucocyte, b 15 mins.; 

 c 30 mins.; d 40 mins.; e 45 mins.; f 50 mins. later than a. From Arnold.^*' 



Dawson,^^* who however did not observe the process in living material. 

 More often, a binucleate cell results from the amitotic division of a 

 nucleus. This event was followed in cultures of heart fibroblasts of the 

 chick embryo by Macklin^^^ ^^^ who found that indentation of the 

 nucleus was sometimes followed by its direct division. In one cell, the 

 extent of the indentation opposite the central body varied over a period 

 of more than two hours, but finally the cleft pushed right through the 

 nucleus and divided it into two. This event is a rare one in such cultures ; 

 it has never been observed by the present writer. In the study by 

 Wermel and Portugalow^^' on the rhythmic growth in size of nuclei in 

 tissue culture, one nucleus which they followed for a period of over five 

 hours finally divided amitotically (Figure 23). Lewis and Webster^ss 

 report that in their cultures of human lymph nodes 'one clear case 

 was seen of amitotic division of the nucleus in one of the large wandering 

 cells', and suggest that multinucleate giant cells may possibly originate 

 by repetition of this process. In order to prove from a series of nuclear 

 outlines in fixed material that direct division is occurring, one must be 



152 



