CELL SIZE AND NUCLEAR SIZE 45 
clear sap increases in amount from zero until it forms the principal 
bulk of the nucleus, and when mitosis comes on it passes into the 
cell body, and as a constituent of the nucleus sinks again to zero. 
The substance which forms the nuclear sap is absorbed by the 
nucleus from the cell body throughout the whole of the resting 
period, only to be thrown out into the cell body again at the end 
of that period. Consequently the nuclear sap is no more a nuclear 
constituent than a protoplasmic one, belonging to both nucleus 
and protoplasm.2. Studies on the growth of nuclear material 
should therefore be confined to the growth of the chromatin, but 
the difficulty of measuring the amount of chromatin at different 
stages will be appreciated without further comment. Also the 
fact that so large a part of the nuclear material belongs also to 
the protoplasm should be taken into account in experiments 
dealing with the isolation of nuclei from protoplasm; evidently 
the only satisfactory way in which such isolation can be accom- 
plished is by isolating chromosomes, rather than resting nuclei. 
There is good reason for believing that the nuclear sap contrib- 
utes to the nourishment and growth of the chromatin and linin, 
and that it in turn receives substances from these, so that the 
materials which pass into the cell body when the nuclear mem- 
brane dissolves, are not wholly the same as those which were 
taken up by the nucleus from the cell body. I have elsewhere 
(02) called attention to the fact that the escaping nuclear sap 
stains more deeply than the cell protoplasm and may therefore 
be called ‘chromatic sap.’ 
As to the mechanism of this intake of protoplasmic substance 
into the nucleus there is every visible evidence that it is of the 
nature of osmosis. The nucleus becomes spherical in shape un- 
less subjected to outside pressure, or to the action of substances 
which cause plasmolysis. The nuclear membrane remains entire 
and distinct until the last phase of nuclear growth, immediately 
preceding mitosis, when the nucleus swells very rapidly and the 
nuclear membrane becomes thin and then disappears. 
The measurements given in the preceding section show that 
the total quantity of the more fluid part of the nucleus, the nuclear 
2Watase (1893) says,—‘‘The structure known as the nucleus contains a great 
deal of cytoplasmic substance.” 
