438 WAEEEN H. LEWIS 



cells soon after division.^ Parallel with the increase in the size 

 of the cell and of its nucleus, there is a diminution in the depth 

 of stain retained by the nucleus. This is very marked if one 

 compares the nucleus of a young daughter cell with that of an 

 old cell just before mitotic changes again take place. Imme- 

 diately after cell division the nuclei are small and deeply stain- 

 ing, because the chromosomes contain but little fluid and the 

 chromatin material which takes up the stain is dense. As the 

 nuclei enlarge through the imbibition of fluid by the chromo- 

 somes, they become less deeply stainable, because the chromatin 

 material becomes less dense. This process of swelling of the 

 chromosomes goes on until a certain metabolic balance is reached 

 and either a more or less stable condition results or there is another 

 complete upset and fluid is again squeezed out of the chromo- 

 somes, leaving them floating in the nuclear sap. During the 

 resting period there is also a supposed gradual increase in the 

 amount of chromatin material, as well as an increase in the size 

 of the nucleus through the imbibition of fluid. If, then, we 

 bear in mind these differences in the normal nuclei, we can 

 explain the differences in the number and size of the nuclear 

 vacuoles and the correlated shrinkage of the nuclei to pycnotic 

 forms in these experiments. In some manner the potassium 

 permanganate caused a contraction of the chromatin material 

 and a squeezing out of the clear nuclear sap in the form of 

 vacuoles. The younger nuclei have less nuclear sap and less 

 chromatin than the older nuclei and were probably the ones 

 which gave rise to the fewest and smallest vacuoles and to the 

 smallest residual chromatin mass (figs. 4, 5, 6, 7, 8, 10). During 

 metaphase, anaphase, and telaphase there are no indications of 

 the formation of these nuclear vacuoles (figs. 1, 2, 3). During 

 the reconstruction period, however, when the chromosomes are 

 supposed to be absorbing fluid and the nucleus increases in size, 

 there was an increase in the amount of vacuolar fluid and in the 

 residual chromatin mass. The older nuclei give rise to a still 

 larger total vacuolar volume and to a larger chromatin residue. 



* Lewis, M. R. and W. H. 1915 Mitochondria and other cytoplasmic struc- 

 tures in tissue cultures. Amer. Jour. Anat., vol. 17, p. 368. 



