THE MITOTIC CYCLE 



their short account of early prophase in the newt erythroblast, these 

 authors speak of the great size of the nucleus, which then nearly fills the 

 cell. It is relatively opaque. Slow movements occur in the cytoplasm, 

 and the nucleus rotates, in one instance through a complete turn in 

 six minutes. Some time after this rotation has ceased, the nuclear 

 membrane suddenly disappears. The longitudinal split in the chromo- 

 somes is then apparent. 



Hughes and Preston^" described the mitosis of a newt liver fibro- 

 blast in tissue culture (Plate VII). Their observations began at a stage 

 when the nucleoli had already disappeared, and the nucleus was filled 

 with granules of comparatively low contrast relative to the nuclear sap. 

 These lay along the developing chromosome threads; gradually the 

 granules seemed to fuse together as more material was deposited on 

 them, and about five minutes after observation had begun, chromo- 

 somes of regular form stood out in clear contrast to their surroundings, 

 and in some their split condition could already be seen. So far, the 

 changes were not accompanied by movement within the nucleus, but 

 from this point the chromosomes began to move, and within three 

 minutes were radially orientated round a clear central space, which 

 was occupied by the developing spindle. There was no definite moment 

 of dissolution of the nuclear membrane. The chromosomes soon began 

 to move to a fresh orientation with the metaphase plate at right angles 

 to the plane of the preparation, and the cell then assumed a more 

 rounded form. The chromosomes contracted in length as they reached 

 the spindle poles, and lost their uniform outlines as they bunched 

 together. Anastomoses between them then were formed. These are 

 clearly seen both in the living fibroblasts and erythrocytes, and also in 

 fixed tissues of the larva of Ambystoma (Dearing^^). These cross- 

 filaments are so regularly arranged, that in the erythrocyte Gomandon 

 et alii^^ speak of 'la disposition caracteristique en damier que prend le 

 resaux chromatique'. To judge from Bearing's drawings this condition 

 persists in a very fine reticular structure of the interphase nucleus. In 

 the succeeding prophase, these fine filaments are gradually resolved. 



Chick and mammalian cells (Plate VIII) — The structure of the inter- 

 phase nucleus of the mouse has already been described (p. 30). In 

 between the nucleoli and the large heterochromatic granules, the nu- 

 cleus is evenly filled with the chromonemata (Plate 11(4) )• ^^ Feulgen 

 preparations of very early prophase these threads are seen to have con- 

 tracted slightly, leaving empty spaces in between them, the large hetero- 

 chromatic aggregates have broken down into smaller units, each of 

 which appears to form part of an individual chromosome. This stage 

 can be recognized by phase-contrast in the living nucleus, for the back- 

 ground of the nucleus is then faintly granular. Where one is fortunate 

 enough to follow with the time-lapse camera a cell into prophase, it can 



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