Page:Popular Science Monthly Volume 83.djvu/191

Rh end it consists of a large vesicle of achromatic substance in which the chromatin usually exists as scattered granules. At the next division some of these granules form chromosomes and all the rest of the nuclear content is liberated into the cell body, to be again absorbed by the daughter nuclei during the succeeding rest period. There is thus a sort of diastole and systole of the nuclear vesicle during every division cycle of a cell, achromatin being taken up by the nucleus during its growth and liberated again into the cell body during its division.

In different cleavage cells of Crepidula plana, when the yolk is eliminated from consideration, the nucleus-plasma ratio varies from 1:0.37 to 1:14.5; that is, the volume of the actual protoplasm in certain cells may be only one third the volume of the nucleus, or in other cells it may be fourteen times that volume, depending largely upon the length of the resting period.

In general the size of a nucleus is directly proportional to the volume of the general protoplasm in the cell, to the length of the resting period, and in cases of abnormal or irregular distribution of chromosomes, to the number and volume of the initial chromosomes which go to form the nucleus. The inciting cause of cell division is not to be found in departures from a normal nucleus-plasma ratio, which is a result rather than a cause of the rate of cell division, but rather in the coincidence of certain metabolic phases in nucleus, centrosome and protoplasm.

If the growth period of the nucleus is very long, the greater part of the protoplasm may be taken into the nucleus, as in those cleavage cells in which the nuclear volume is about three times as great as that of the protoplasm outside of the nucleus; if the growth period of the nucleus is short, the nucleus remains correspondingly small. If nuclear division is prevented by hypertonic solutions or by decreased oxygen tension, the nuclei may grow to an enormous size until they contain the greater part of the cell protoplasm.

In certain stages of the division cycle it is possible by the use of hypertonic solutions to prevent the daughter chromosomes from absorbing achromatin, and in such cases these chromosomes form small, densely chromatic nuclei, while the achromatin may be gathered into one or many vesicles. In other cases, the chromatin may be caused to contract and to squeeze out the achromatin. The latter case is similar to that which takes place normally in the formation of a spermatozoon from a spermatid, where there is a condensation of the chromatin of the spermatid nucleus and a squeezing out of the achromatin; this diminution of the nucleus is coincident with the transformation of the protoplasm of the spermatid into differentiation products. A similar thing happens in superficial epithelial cells which are undergoing