PROTOZOA 833 as compared with what previously obtained. Now the two groups diverge, and in many cases a striated appear ance of the achromatin substance between the two groups of loops of chromatin is observable (H). In some cases (especially egg-cells) this striatei arrangement of the achromatin substance precedes the separation of the loops (G). The striated achromatin is then termed a " nucleus- spindle," and the group of chromatin loops (Fig. I., G, a) FIG. I. Tviiiyokinesis of a typical tissue-cell (epithelium of Salamander) after Flamming and Klein. The series from A to 1 represent the succ ssive stages in the movement of the chromatin fibres during division, excepting G, which represents the " nucleus-spindle" of an egg-cell. A, resting nucleus; D, wivath- form; E, single star, the loops of the wreath being broken; K, separation of the star into two groups of U-shaped fibres; II, diaster or double star; I, comple tion of the cell-division and formation of two resting nuclei. In G the chromatin fibres are marked a, and correspond to the phase shown in F ; they are in this case called the "equatoiial plate"; 6, achromatin fibres forming the nucleus-spindle ; c, granules of the cell-protoplasm forming a "polar star." Such a polar star is seen at each end of the nucleus-spindle, and is not to be confused with the diaster II. is known as " the equatorial plate." At each end of the nucleus-spindle in these cases there is often seen a star consisting of granules belonging to the general proto plasm of the cell (G, c). These are known as " polar stars." After the separation of the two sets of loops (H) the protoplasm of the general substance of the cell becomes constricted, and division occurs, so as to include a group of chromatin loops in each of the two fission products. Each of these then rearranges itself together with the associated achromatin into a nucleus such as was present in the mother-cell to commence with. This phenomenon is termed " karyokinesis," and has been observed, as stated above, in a large variety of cells constituting tissues in the higher animals and plants. There is a tendency among histologists to assume that this process is carried out in all its details in the division of all cells in the higher plants and animals, and accordingly to assume that the structural differentiation of achromatin plasma and chromatin nucleus-fibres exists in the normal nucleus of every such cell. If this be true, it is necessary to note very distinctly that the nucleus of the Protozoon cell-individual by no means conforms universally to this model. As will be seen in the sequel, we find cases in which a close approach is made by the nucleus of Protozoa to this structure and to this definite series of movements during division (Fig. VIII. 3 to 12, and Fig. XXV.); and a knowledge of these phenomena has thrown light upon some appearances (conjugation of the Ciliata) which were previously misinterpreted. But there are Protozoa with a deeply-placed nucleus-like structure which does not pre sent the typical structure above described nor the typical changes during division, but in which on the contrary the nucleus is a very simple homogeneous corpuscle or vesicle of more readily stainable protoplasm. The difficulties of observation in this matter are great, and it is proportionately rash to generalize ; but it appears that we are justified at the present moment in asserting that not all the cells even of higher plants and animals exhibit in full detail the structure and movement of the typical cell-nucleus above figured and described ; and accord ingly the fact that such structure and movement cannot always be detected in the Protozoon cell-nucleus must not be regarded as either an isolated phenomenon peculiar to such Protozoon cells, nor must it be concluded that we have only to improve our means of analysis and observation in order to detect this particular structure in all nuclei. It seems quite possible and even probable that nuclei may vary in these details and yet be true nuclei. Some nuclei which are observed in Protozoon cell-bodies may be regarded as being at a lower stage of differentiation and special iza tion than are those of the epithelial and embryonic cells of higher animals which exhibit typical karyokinesis. Others on the contrary, such as the nuclei of some IJadiolaria (vide infra), are probably to be regarded as- more highly developed than any tissue cell-nuclei, and will be found by further study to present special phenomena peculiar to themselves. In some of the highest Protozoa (the Ciliata) it has lately been shown that the nucleus may have no existence as such, but is actually dispersed throughout the protoplasm in the form of fine particles of chromatin-substance which stain on treatment with car mine but are in life invisible (84). This diffuse condition of the nuclear matter has no parallel, at present known, in tissue-cells, and curiously enough occurs in certain genera of Ciliata whilst in others closely allied to them a solid single nucleus is found. The new results of histological research have necessitated a careful study of the nucleus in its various stages of growth and division in the cell- bodies of Protozoa and a comparison of the features there observed with those established as " typical " in tissue-cells. Accordingly we have placed the figure and explanation of the typical cell-nucleus in the first place in this article for subsequent reference and comparison. CORTICAL SUBSTANCE. The superficial protoplasm of an embryonic cell of an Enterozoon in the course of its development into a muscular cell undergoes a change which is paralleled in many Protozoa. The cortical layer becomes dense and highly refringent as compared with the more liquid and granular medullary substance. Probably this is essentially a change in the degree of hydration of the protoplasm itself, although it may be accompanied by the deposition of metamorphic products of the protoplasm which are not chemically to be regarded as protoplasm. The differentiation of this cortical substance (which is not a frequent or striking phenomenon in tissue-cells) may be regarded as an ectoplastic (i.e., peripheral) modification of the protoplasm, comparable to the entoplastic (central) modification which produces a nucleus. The formation of " cortical substance " in the Protozoa furnishes the basis for the most important division into lower and higher forms, in this assemblage of simplest animals. A large number (the Gymnomyxa) form no cortical substance ; their protoplasm is practically (except ing the nucleus) of the same character throughout. A nearly equally large number (the Corticata) develop a complete cortical layer of denser protoplasm, which is distinct from the deeper medullary protoplasm. This layer is permanent, and gives to the body a definite shape and entails physiological consequences of great moment. The cortical protoplasm may exhibit further specialization of structure in connexion with contractile functions (muscular). ECTOPLASTIC PRODUCTS CHEMICALLY DISTINCT FROM PROTOPLASM. The protoplasm of all cells may throw down as a molecular precipitate distinct from itself chemical compounds, such as chitin and horny matter and other nitrogenized bodies, or again non-nitrogenous compounds, such as cellulose. Very usually these substances are deposited not external to but in the superficial proto-
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