AMITOSIS IN CELLS GROWING IN VITRO. 453 



6.50 P.M. (Fig. 20). The cell is even smaller than before, and more rounded. 

 In the clear area within it is a refractive bar, which proves to be the equatorial 

 plate of chromosomes. Individual chromosomes cannot be distinguished, so that 

 it is impossible to count them, but their ends may be seen, as they project towards 

 the poles of the spindle. The chromosomes show a slow, oscillatory type of move- 

 ment, slight in extent. The spindle is represented by a fairly clear area, shaped 

 like two cones base to base, at the extremities of which the centrosomes are situated. 

 Astral rays cannot be seen. Mitochondria and fat globules encase this central 

 area containing the spindle like a shell. This becomes evident by focusing up and 

 down. Though the uppermost part of the spherical cell is on a level with the flat 

 resting cells, which are to be found about it, the lowermost part is much below this, 

 due to the fact that the cell is thickened, and projects into the medium. 



7.05 P.M. (Fig. 21). The cell has suddenly become elongated and constricted 

 at the equator: the plate of chromosomes has evidently split, and the anaphase of 

 mitosis is being witnessed. The constriction about the middle of the cell can be 

 seen to be increasing, causing streaming of globules toward its extremities. At 

 the same time small, bubble-like processes emerge from the borders of the cell, 

 seeming to be forced out by the internal pressure of the cell body. Into these pro- 

 tuberances granules flow but later return into the main mass of the cytoplasm. 

 These processes soon become flattened and extended, forming pseudopodia possessed 

 of hyaline borders with amoeboid movement. The individual chromosomes of the 

 two masses in the expanded extremities gradually lose their distinctness and 

 become dispersed. 



7.25 P.M. (Fig. 22). The constriction is more marked and the cells are almost 

 entirely separated. They are also becoming flattened out. In the upper daughter 

 cell a clear space for the nucleus is appearing. The constricted zone is somewhat 

 more highly refractive than the surrounding tissue and resembles a short thread. 

 This probably contains part of the remains of the spindle. Nuclear details are 

 not yet visible. 



As the cell is watched nucleoli become manifest, at 7.35 P.M. two of these being 

 seen in each daughter nucleus in a clear space, surrounded by a distinct nuclear 

 membrane. 



8.00 P.M. (Fig. 23). The daughter cells are now almost of normal size. Each 

 nucleus has a concave side, as is usual, and in each concavity is the new centre- 

 sphere, from which the mitochondria radiate. Two nucleoli appear in each nu- 

 cleus. The cells are spread out and flattened, and the fat granules are disposed as 

 in the ordinary cell. 



The entire observation covered eight hours. The more active part of mitotic 

 division occupied about two hours, but if the initial nuclear changes be included 

 the duration of mitosis is much longer. 



The sequence of changes followed in the above cell are in 

 almost all respects similar to those of mitosis many times ob- 

 served in the mononucleate cell. The only difference is that in 

 the cell described there were to begin with two separate nuclear 

 sacs instead of one. From the presence, in fixed preparations, of 

 bipartite nuclei each portion of which is in a condition of spireme, 

 and the absence, from such preparations, of bipartite nuclei in 



