But all these powers are exercised at the cost of a chemical transformation or degradation of the protoplasm itself, in part respiratory, i. e. oxydative. The products of this degradation, Carbon dioxide and various nitrogenous compounds, are useless to the organism, and are excreted. The whole of the vital properties enumerated can be exercised only while the protoplasm is saturated with water. One of the consequences of the vital energies of protoplasm is that as a substance it can never be obtained in a chemically pure condition, which is only approached when it is starved. Otherwise it is laden with the products of progressive and regressive metamorphosis. It may be added that the protoplasm of a cell often gives origin by conversion to an external or internal cell-skeleton, the characters of which, both chemical and physical, are extremely variable1.

1 See Maupas, C. R. 88, I879, p. I274.

The nucleus of a cell is a structure sharply marked off from the protoplasm. In its simplest state it is homogeneous and more or less rounded, but capable sometimes of changes of shape. It consists principally of a substance termed nuclein from a chemical point of view, or chromatin from its marked physical peculiarity, that of readily absorbing, and to a much greater degree than the rest of the cell, various kinds of stains such as carmine. But this simple structure is rarely retained. The nucleus is limited externally by a nuclear membrane, its chromatin is disposed in very various ways, as a reticulum, a much coiled thread, in fragments as a lining to the nuclear membrane and one or more central spots. To nodal thickenings of the thread, etc, or to the fragments of chromatin, the term nucleolus is generally applied. The intervals between the chromatin elements are occupied by a nuclear fluid, composed of a protoplasm (caryoplasma), which may be resolved like the protoplasm of the cell (of which it is a part) into a denser and more fluid portion. As this protoplasm takes up stains but slightly, it is frequently designated achromatin.

The structure of the nucleus is not always the same at all periods of its life; it may be at first homogeneous, but as a rule assumes one of the more complex forms.

1 See the introductory chapter in Foster's 'Textbook of Physiology,' and on the movements, etc. of protoplasm, Engelmann, 'Die Protoplasma- und Flimmerbewegung,' in Hermann's Handbuch der Physiologie, i. I879, P. 343 et seqq., the first part of which is translated in Q. J. M. xxiv. I884. On the structure and physiology of ciliated cells, see also Engelmann, Pfliiger's Archiv fiir Physiologie, xxiii. I880.

The process of division of the cell may be comparatively simple or complicated. The division of the protoplasm is preceded, or accompanied, by division of the nucleus. The latter process may be direct or amitotic, the nucleus simply elongating, and being split by a constriction. Or it may be indirect or mitotic, the achromatin being disposed in lines parallel to the long axis of the nucleus, making the figure of a spindle, and the chromatin grouped at the centre of the spindle, dividing into parts which move in opposite directions to either pole of the spindle, whilst a constriction splits the nucleus in two. It is rare for the chromatin to be grouped in two masses on the equator and the split of the nucleus to take place through its poles. The figures seen in the process are spoken of as karyokinetic. It has been found that the typical mitotic and amitotic modes of divisions are connected by intermediate phases, at least in some tissue cells. The nuclear membrane is dissolved in mitosis and reconstituted round the new nuclei1.

1 The denser mitome of the nucleus and the body of the cell may give rise to an equatorial plate, or the former may do so, and not the latter. This plate, which is common in plants, but has only been detected in certain tissue-cells of Arthropoda, may evanesce, or fission may take place through its median plane. It may be noted that the mitome of the cell-body is frequently arranged in radii during the nuclear changes, and that a clear spot, the polar spot or corpuscle, may appear at each pole of the spindle. For the structure of the cell, see Carnoy, 'La Biologie cellulaire,' Lierre, Fasc. i. pt. 2, I884; for cell-division (cytodieresis) in Arthropoda, Id. 'La Cellule/ Lierre, i. 1885-6, and the summary of both papers by A. Bolles Lee in Q. J. M. xxvi. I886, p. 481; cf. also Flemming and Carnoy in Z. A. ix. I886. For the fission of the giant-cells, etc. in the medulla of bone, see Denys, 'La Cellule,' ii. (2), I887.

The single cell from which a multicellular animal is developed is known as an ovum. It may be derived from an epithelium or sub-epithelium, ectodermic or endodermic (Coelenteratd); or from a special organ, the ovary, furnished with a duct and developed usually in the mesoblast, sometimes from special cells set apart at a very early stage of development, e. g. in some Insecta, perhaps in rare instances from the endoderm (some Turbellarid). It may be naked, or provided with one or more envelopes, derived from itself, from surrounding cells, or special glands1. It may be hyaline, or it may be filled to a greater or less extent with nutrient material, derived by its own vital energies from the lymph-plasma of the body, from the products produced by the regressive changes of surrounding cells (granulosa cells) of the ovary, rarely from other cells. This nutrient reserve-material may be distinguished as food-yolk or deutoplasm from the protoplasm with which it is mixed2. Or the nutrient material may be derived from a special gland, the vitellarium, and be inclosed with the ovum in the egg-shell to be utilised as the ovum segments (some Turbellaria, Trematoda, Cestoda) 3. In one phase of the life-history of the digenetic Trematoda, the Sporocyst or Redia, the reproductive cell is one of a number of cells filling the central part of the body and lining the body walls.