C Y T O L O G Y substance containing drops of oil. They are stained deep cal or slightly irregular bodies, the nucleoli. In the living red in dilute solution of alkannin. There appears to be condition the nucleus appears granular, and the thread one in each cell, and in Orithogalum umbellatum it occurs structure is in most cases invisible, but the nucleolus is at one of the poles of the elongated nucleus. An oily sometimes distinguishable by its more refractive appearsubstance in the form of drops may also occur in chromato- ance. The structure of the nuclear thread can only be phores, and, in the Hepaticse, oil-bodies are found which made out by the use of reagents. It consists of a groundmay have some relation to elaioplasts. Although no special substance called Linin, which is only slightly stained by the oil-forming plastid has been observed, it appears from these ordinary nuclear stains, and, embedded in it, numerous observations that oil may sometimes be formed by special grains of a substance called Chromatin, having a strong structures, or may accumulate in them (Pffeffer). The eye- affinity for stains. These chromatin-granules are looked spot which is found in motile cells of the Algae, Protococ- upon by some observers as independent morphological units, cus, Euglena, &c., may also have something in common the ids of Weismann. The staining reactions of the various with the elaioplast. In Euglena it consists of a proto- parts of the nucleus depend to some extent upon their plasmic substance, in which are embedded numerous chemical constitution, especially with regard to the amount spherical or irregular drops of a red oily substance. The of phosphorus they contain. The chromatin is practically red colouring matter turns blue in concentrated sulphuric identical with nuclein, a complex albuminoid compound acid, and is probably related to the red colouring matter containing about 3 per cent, of phosphorus. This has of chromoplasts. The eye-spot appears to be concerned a strong attraction for basic aniline dyes, and can usually in some way with the response to the light-stimulus be distinguished from those parts of the cell which are more easily coloured by acid anilines. But the staining which is exhibited by cells containing it. It is highly probable that starch is only produced as reactions of nuclei may vary at different stages of their the result of the activity of chromatophores, either in development; and it is probable that there is no method connexion with chromoplasts, chloroplasts, or leucoplasts. of staining which differentiates with certainty the various Starch exists, in the majority of cases, in the form of morphological constituents of the nucleus. The formation of new cells by division is, in the case grains, which are composed of stratified layers arranged around a nucleus or hilum. The stratification, which of uninucleate cells, preceded by or accompanied by the may be concentric or excentric, appears to be due to a division of the nucleus. In multinucleate cells the division difference in density of the various layers. The outer of the nucleus is independent of the division of the cell. layers are denser than the inner, the density decreasing Nuclear division may be indirect or direct, that is to say more or less uniformly from the outside layers to the it may either be accompanied by a series of complicated centre or hilum. The outermost, newly-formed layer is changes in the nuclear structures called mitosis or karyocomposed of a more homogeneous, denser substance than kinesis,ovit may take place by simple direct division, amitosis, the inner one, and can be distinguished in all starch-grains or fragmentation. Direct division is a much less common that are in process of development. The separate layers phenomenon than was formerly supposed to be the case. of the starch-grain are deposited on it by the activity of It occurs most frequently in old cells, or in cells which the chromatophore, and according to Meyer the grain is are placed under abnormal conditions. It may also take always surrounded by a thin layer of the chromatophore place where rapid proliferation of the cell is going on, as which completely separates it from the cytoplasm. The in the budding of the Yeast plant. It takes place in the layers appear to be made up of elements which are internodal cells of Characese; in the old internodal cells of arranged radially. These are, according to Meyer, Tradescantia; and in various other cells which have lost acicular crystals, which he calls trichites. The starch their power of division. It has been shown that, in cells grain may thus be regarded as a crystalline structure of of Spirogyra placed under special conditions, amitotic the nature of a sphere-crystal, as has been suggested by division can be induced, and that normal mitosis is resumed when they are placed again under normal conditions. many observers. The nucleus appears to be an essential constituent of Amitosis is probably connected by a series of intermediate the cell. Its presence has been demonstrated in all plants gradations with karyokinesis. In indirect nuclear division the nucleus undergoes a series with the and exception Cyanophyceae and Nucleus, -gsieteria, even of herethestructures have been of complicated changes, which result in an equal division observed which resemble nuclei in some of their char- of the chromatic substance between the two daughter acteristics, and are thought to be such by some observers. nuclei. Four stages can be recognized. (1) Prophase.— The nucleus is generally regarded as a controlling centre The nucleus increases in size ; the network disappears, and of cell-activity, upon which the growth and development a much convoluted thread takes its place (Fig. 1, B). The of the cell in large measure depends, and as the agent by chromatin substance increases in amount; the thread stains which the transmission of specific qualities from one more deeply, and in most cases, presents a homogeneous generation to another is brought about. If it is absent, appearance. This is commonly called the spirem-figure. the cell loses its power of assimilation and growth, and soon The chromatin thread next becomes shorter and thicker, dies. Haberlandt has shown that in plant cells, when any the nucleoli begin to disappear, and the thread breaks up new formation of membrane is to take place in a given spot, into a number of segments—chromosomes—which vary in the nucleus is found in its immediate vicinity; and Klebs number in different species, but are fairly constant in the found that only that portion of the protoplasm of a cell same species (Fig. 1, C, D). Coincident with these changes which contains the nucleus is capable of forming a cell- the nuclear membrane disappears and a spindle-shaped or wall ; whilst Townsend has further shown that if the non- barrel-shaped group of threads makes its appearance in the nucleated mass is connected by strands of protoplasm to midst of the chromosomes, the longitudinal axis of which the nucleated mass, either of the same cell or of a neighbour- is at right angles to the plane of the division (Fig. 1, F). At each pole of this spindle figure there often occur fibres ing cell, it retains the power of forming a cell-membrane. The nucleus consists of an irregular network of threads radiating in all directions into the cytoplasm, and somesurrounded by a nuclear membrane, which separates it from times a minute granular body, the centrosome, is also found the cytoplasm. The nuclear cavity in which the network is there. (2) Metaphase.—The chromosomes pass to the placed contains a more or less fluid nuclear sap, and in the equator of the spindle and become attached to the spindlemajority of plant-nuclei, but not in all, one or more spheri- fibres in such a way that they form a radiating star-shaped 334
