has the power of absorbing certain kinds of stains very actively, and is consequently deeply stained when treated as the microscopist commonly prepares his specimens.  For this reason it has been named chromatin (Fig, 23, e), although in more recent times other names have been given to it.  Of all parts of the cell this chromatin is the most remarkable.  It appears in great variety in different cells, but it always has remarkable physiological properties, as will be noticed presently.  All things considered, this chromatin is probably the most remarkable body connected with organic life.

[Illustration:  FIG. 26.—­Different forms of nucleii.]

The nucleii of different animals and plants all show essentially the characteristics just described.  They all contain a liquid, a linin network, and a chromatin thread or network, but they differ most remarkably in details, so that the variety among the nucleii is almost endless (Fig. 26).  They differ first in their size relative to the size of the cell; sometimes—­especially in young cells—­the nucleus being very large, while in other cases the nucleus is very small and the protoplasmic contents of the cell very large; finally, in cells which have lost their activity the nucleus may almost or entirely disappear.  They differ, secondly, in shape.  The typical form appears to be spherical or nearly so; but from this typical form they may vary, becoming irregular or elongated.  They are sometimes drawn out into long masses looking like a string of beads (Fig. 24), or, again, resembling minute coiled worms (Fig. 21), while in still other cells they may be branching like the twigs of a tree.  The form and shape of the chromatin thread differs widely.  Sometimes this appears to be mere reticulum (Fig. 23); at others, a short thread which is somewhat twisted or coiled (Fig. 26); while in other cells the chromatin thread is an extremely long, very much twisted convolute thread so complexly woven into a tangle as to give the appearance of a minute network.  The nucleii differ also in the number of nucleoli they contain as well as in other less important particulars.  Fig. 26 will give a little notion of the variety to be found among different nucleii; but although they thus do vary most remarkably in shape in the essential parts of their structure they are alike.

==Centrosome.==—­Before noticing the activities of the nucleus it will be necessary to mention a third part of the cell.  Within the last few years there has been found to be present in most cells an organ which has been called the centrosome. This body is shown at Fig. 23, g.  It is found in the cell substance just outside the nucleus, and commonly appears as an extremely minute rounded dot, so minute that no internal structure has been discerned.  It may be no larger than the minute granules or microsomes in the cell, and until recently it entirely escaped the notice of microscopists.  It has now, however,