THE NUCLEUS A fragment of a cell deprived of its nucleus may live for a considerable time and manifest the power of co-ordinated movement without perceptible impairment. Such a mass of protoplasm is, however, devoid of the powers of assimilation, growth, and repair, and sooner or later dies. In other words, those functions that involve destructive metabolism may continue for a time in the absence of the nucleus ; those that involve constructive metabolism cease with its removal. There is, therefore, strong reason to believe that the nucleus plays an essential part in the constructive metabolism of the 1 The following list includes only some of the various names that have been given to these hypothetical units by modern writers : Physiokrical units (Spencer); vmmults (Darwin) ; tangens (De Vries); (Wiesner); micill,r ( \igen); flit/tiler (Haeckel and Elssberg) ; inotagmata (Engelmann); biothores (Weismann); biobiasts (Beale); somacules (Foster); idioblasts (Hertwig); idiosomes (Whitman); bi‘{'ens (Verworn); micro:ymas (1lechamp and Estor); Aremmi. (Haacke).
cell, and through this is especially concerned with the formative processes involved in growth and development. For these and many other reasons, to be discussed hereafter, the nucleus is generally regarded as a controlling centre of cell-activity, and hence a primary factor in growth, development, and the transmission of specific qualities from cell to cell, and so from one generation to another.
I. General Structure The cell-nucleus passes through two widely different phases, one of which is characteristic of cells in their ordinary or vegetative condition, while the other only occurs during the complicated changes involved in cell-division. In the first phase, falsely characterized as the " resting state," the nucleus usually appears as a rounded sac-like body surrounded by a distinct membrane and containing a conspicuous irregular network (Figs. 5, 7, 10). Its form, though subject to variation, is on the whole singularly constant, and shows no definite relation to that of the cell in which it lies. Typically spherical, it may, in certain cases, assume an irregular or amceboid form, may break up into a group of more or less completely separated lobes (polymorphic nuclei), or may be perforated to form an irregular ring (Fig. II, D). It is usually very large in gland-cells
and others that show a very active metabolism, and in such cases its surface is sometimes increased by the formation of complex branches ramifying through the cell (Fig. it, E). Interesting modifications of the nucleus occur in the unicellular forms. In the ciliate Infusoria the body contains nuclei of two kinds, viz. a large macronucleus and one or more smaller micronuclei. The first of these shows a remarkable diversity of structure in different forms, being often greatly elongated and sometimes showing a moniliform structure like a string of beads. In Trachelocerca and some other Infusoria, according to Gruber ('84), the nucleus is not a single definite body, but is represented by minute granules scattered throughout the cell-substance (Fig. 12) ; Butschli describes somewhat similar diffused nuclei in some of the Flagellates, and in the Bacteria.
In the ordinary forms of nuclei in their resting state the following structural elements may as a rule be distinguished (Figs. 5,6, 7, lo, 11):— a. The nuclear membrane, a well-defined delicate wall which gives the nucleus a sharp contour and differentiates 'it clearly from the surrounding cytoplasm.
h. The nuclear reticulum. This, the most essential part of the nucleus, forms an irregular branching network or reticulum which consists of two very different constituents. The first of these, the nuclear substance par excellence, is known as chromatin (Flemming) on account of its very marked staining capacity when treated with various dyes. In some cases the chromatin forms a nearly continuous network, but it often appears in the form of more or less detached rounded granules or irregular bodies. The second constituent is a transparent substance, invisible until after treatment by reagents, known as linin (Schwarz). This substance, which is probably of the same nature as the cytoplasmic network outside the nucleus, surrounds and supports the chromatin, and thus forms the basis of the nuclear network.