Centrosome.The centrosome is a minute homogeneous granule found in the cytoplasm of some cells in the neighborhood of the nucleus.
The protoplasm of a living cell con.sists of a semifluid granular substance, called the cytoplasm, one or more nuclei, and sometimes centrosomes and plastids.
Chromatophores.The chromatophores or plastids are protoplasmic structures, denser than the cytoplasm, and easily distinguishable from it by their color or greater refractive power.
In Spirogyra the pyrenoids are distinctly connected by cytoplasmic strands to the central mass of cytoplasm, which surrounds the nucleus, and according to some observers, they increase exclusively by division, followed by a splitting of the cytoplasmic strands.
The spindle arises partly from the cytoplasm, partly from the nucleus, or it may be derived entirely from the nucleusintranuclear spindleas occurs in many of the lower plants (Fungi, &c.).
At each pole of this spindle figure there often occur fibres radiating in all directions into the cytoplasm, and sometimes a minute granular body, the centrosome, is also found there.
In a few cases both among the higher and the lower plants, of which the formation of spores in the ascus is a typical example, new cells are formed by the aggregation of portions of the cytoplasm around the nuclei which become delimited from the rest of the cell iontents by a membrane.
Usually the cytoplasm shows a marked affinity for the acid stains, but the different bodies found in the cell may show great variation in their staining reactions.
Evidence is not wanting, however, that the cytoplasm must be regarded as, fundamentally, a semifluid, homogeneous substance in which by its own activity, granules, vacuoles, fibrils, &c., can be formed as secondary structures.
As division proceeds, the filamentous nature of this cytoplasm becomes more prominent and the threads begin either to converge towards the poles of the nucleus, to form a bipolar spindle, or may converge towards, or radiate from, several different points, to form a multipolar spindle.
In the Thallophytes the cytoplasm may be segmented by constriction, due to the in-growth of a new cell wall from the old one, as in Spirogyra and Cladophora, or by the formation of cleavage furrows in which the new cell-wall is secreted, as occurs in the formation of the spores in many Algae and Fungi.
The cell body, or cytoplasm, is apparently composed of a fine reticulum or network, containing within the meshes a soft viscid, transparent substance, the cell-sap, or hyaloplasm, which is probably a nutrient material to the living cell.
The cytoplasm is largely concerned in the formation of spindle fibres and centrosomes, and such structures as the cell membrane, cilia, or flagella, the coenocentrum, nematoplasl~ or vibrioids and physodes are also products of its activity.
In some cases the region where the penetration of the male organ takes place is indicated on the oosphere by a hyaline receptive spot (Oedogonium, Vaucheria, &c.), or by a receptive papilla consisting of hyaline cytoplasm (Peronosporeae).
The chromatin is distributed throughout the cytoplasm in the form of granules which may be regarded as a distributed nucleus corresponding to what Hertwig has designated, in protozoa, chromidia.
Varies in shape, but is usually round or oval, and is sharply defined by a nuclear membrane from the cytoplasm in which it lies.
In the Algae, such as Fucus, Volvox, Oedogonium, Bulbochaete, and in the Fungus Monoblepharis, the spermatozoid is a small oval or elongate cell containing nucleus, cytoplasm and sometimes plastids.
The strongest direct evidence seems to be that the nuclear substances are the only parts of the cells which are always equivalent in quantity, and that in the higher plants and animals the male organ or spermatozoid is composed almost entirely of the nucleus, and that the male nucleus is carried into the female cell without a particle of cytoplasm.i Since, however, the nucleus of the female cell is always accompanied by a larger or smaller quantity of cytoplasm, and that in a large majority of the power plants and animals the male cell also contains cytoplasm, it cannot yet be definitely stated that the cytoplasm does not play some part in the process.
It consists in an unequal number of chromosomes passing over to each of the daughter nuclei, so that one may become hypochromatic, the other hyperchromatic. When this happens the resulting cleavage of the cytoplasm and nucleus is also unequal.
In some cases both the nucleus and the chromatophores may be carried along in the rotating stream, but in others, such as T.Titeila, the chloroplasts may remain motionless iii a non-motile layer of the cytoplasm in direct contact with the cell wall.i Desmids, Diatoms and Oscillaria show creeping movements probably due to the secretion of slime by the cells; the swarmspores and plasmodium of the Myxomycetes exhibit amoehoid movements; and the motile spores of Fungi and Algae, the spermatozoids of mosses, ferns, &c., move by means of delicate prolongations, cilia or flagella cf the protoplast.