[Illustration:  FIG. 23.—­A cell as it appears to the modern microscope. a, protoplasmic reticulum; b, liquid in its meshes; c, nuclear membrane; d, nuclear reticulum; e, chromatin reticulum; f, nucleolus; g, centrosome; h, centrosphere; i, vacuole; j, inert bodies.]

==Structure of Protoplasm==.—­The structure of protoplasm is not yet thoroughly understood by scientists, but a few general facts are known beyond question.  It is thought, in the first place, that it consists of two quite different substances.  There is a somewhat solid material permeating it, usually, regarded as having a reticulate structure.  It is variously described, sometimes as a reticulate network, sometimes as a mass of threads or fibres, and sometimes as a mass of foam (Fig. 23, a).  It is extremely delicate and only visible under special conditions and with the best of microscopes.  Only under peculiar conditions can it be seen in protoplasm while alive.  There is no question, however, that all protoplasm is permeated when alive by a minute delicate mass of material, which may take the form of threads or fibres or may assume other forms.  Within the meshes of this thread or reticulum there is found a liquid, perfectly clear and transparent, to whose presence the liquid character of the protoplasm is due (Fig. 23, b).  In this liquid no structure can be determined, and, so far as we know, it is homogeneous.  Still further study discloses other complexities.  It appears that the fibrous material is always marked by the presence of excessively minute bodies, which have been called by various names, but which we will speak of as microsomes.  Sometimes, indeed, the fibres themselves appear almost like strings of beads, so that they have been described as made up of rows of minute elements.  It is immaterial for our purpose, however, whether the fibres are to be regarded as made up of microsomes or not.  This much is sure, that these microsomes —­granules of excessive minuteness—­occur in protoplasm and are closely connected with the fibres (Fig. 23, a).

==The Nucleus.==—­(a) Presence of a Nucleus.—­If protoplasm has thus become a new substance in our minds as the result of the discoveries of the last twenty years, far more marvelous have been the discoveries made in connection with that body which has been called the nucleus.  Even by the early microscopists the nucleus was recognized, and during the first few years of the cell doctrine it was frequently looked upon as the most active part of the cell and as especially connected with its reproduction.  The doctrine of protoplasm, however, so captivated the minds of biologists that for quite a number of years the nucleus was ignored, at least in all discussions connected with the nature of life.  It was a body in the cell whose presence was unexplained and which did not fall into accord with the general view of protoplasm as the physical basis of life.  For a while, therefore, biologists gave little attention to it, and were accustomed to speak of it simply as a bit of protoplasm a little more dense than the rest.  The cell was a bit of protoplasm with a small piece of more dense protoplasm in its centre appearing a little different from the rest and perhaps the most active part of the cell.