Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/14

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NIMAL HISTOLOGY (from, a web or tissue, and , discourse) is the study of the minute structure of the tissues of animals. By a tissue is meant any part of an organism which has undergone special changes in structure in adaptation to the performance of special functions. These special changes are expressed by the general term "differentiation." In the lowest animal organisms, the whole of whose bodies are composed of the undifferentiated living substance termed "protoplasm," we find all its functions shared by every part of the organism. An amœba, for example, it is well known, is capable of finding, seizing, devouring, digesting, and assimilating food, has a special provision for collecting fluid and pumping it out of its body, respires by its whole surface, moves about apparently where it will, exhibits a sensibility to tactile impressions, and reacts in all probability to smell if not to sound and light,—in short, is capable of performing, although with the lowest possible amount of activity, almost every function which animals vastly higher in the scale of organization exhibit. But even in the amœba we cannot say certainly that there is no differentiation of its protoplasm. For a condensed portion—the nucleus—is set aside to initiate the reproductive function, and it is by means of the external and firmer layer (ectoplasm) that its movements are effected and its relations with the external world maintained, while the internal more fluid protoplasm (endoplasm) is concerned with the digestion of the food. Still there are simple organisms whose protoplasm is probably absolutely undifferentiated. On the other hand, there are other organisms which are also regarded as composed of simple protoplasm, and are constituted by a single cell, which nevertheless show a marked progress in the differentiation of portions of their substance apart altogether from the presence of a nucleus. Such differentiation in unicellular organisms generally takes the form of the production of a shell or "test," as in the Foraminifera and in Noctiluca, which subserves purely passive functions of sustentation or defence. It is not certain in such cases whether the structure thus produced is formed by the direct conversion of the protoplasm or by an exudation on the surface which subsequently hardens. But portions of the protoplasm may be set aside for the performance of active functions. We see this in its production in the form of locomotory organs, either temporary (pseudopodia) or permanent (cilia). But in neither of these can any actual change in the minute structure of the protoplasm be observed. A differentiation does, however, occur in one remarkable instance—the flagellum, namely, of the Noctilucidæ (fig. 1), which exhibits as definite a transverse striation as does the cross-striated or voluntary muscular tissue of the higher animals, in which structural peculiarity it is impossible not to infer a relation to its contractile functions; and similarly, in the Vorticellidæ, there is a differentiation of the protoplasm of the rapidly contractile stalk.

Whereas in the more highly organized unicellular animals portions of the single cell are thus set aside for the performance of special functions, and modified in structure accordingly, in multicellular animals, on the other hand, we find whole cells and sets of cells set apart and differentiated. It is to such modifications in sets of cells in multicellular organisms, rather than in portions of the protoplasm of a unicellular organism, that "histological differentiation" is commonly restricted; and each such set of cells, destined for the performance of a special function, and modified accordingly in structure, is denominated a "tissue."

The animal tissues may be classed under the four heads of Epithelium, Connective Tissue, Muscular Tissue, and Nervous Tissue. Of these four classes of tissue the epithelium is the most primitive and least altered. In the development of the Metazoa the numerous embryonic cells which result from the division of the single cell—the ovum—tend in nearly every case to arrange themselves as a single layer surrounding a central cavity (unilaminar condition of the blastoderm), (fig. 2, A). Presently a part of the wall of the hollow sphere becomes invaginated, so that, instead of a vesicle enclosed by but a single layer of cells, a cup (Gastrula, Haeckel), is produced (fig. 2, B), the wall of which is formed by two layers derived from the original single layer, and separated from one another by a narrow interval (which is all that remains of the original cavity of the vesicle) except at the orifice of the cup, where they are continuous (bilaminar condition of the blastoderm). 1.—Flagellum of Noctiluca miliaris (highly magnified). s, transversely striated substance; b base of attachment to body of animal. 2.—Sections through the unilaminar (A), bilaminar (B), and trilaminar (C) conditions of the typical blastoderm. Ect., ectoderm; Ent., entoderm; Mes., mesoderm; c, primitive connective tissue. At this part some cells become separated from one or both of these two primary layers, and, extending in and occupying the cleft-like space which separates them, become a third layer of cells, which differs from those of the other two in not being arranged into a continuous membrane, and not, therefore, forming an epitheliam (trilaminar condition of the blastoderm), (fig. 2, C). Now, of these three layers, the outer one, or ectoderm, and the inner one, or entoderm, give rise to all the epithelial tissues of the body. The nervous tissues are also derived from the ectoderm; whereas the connective and muscular tissues originate in the mesoderm or middle layer. In most of the Cœlenterata, however, the mesoderm is not developed at one part only of the embryo as in the higher Metazoa. In the hydroid polyps and Medusæ it never becomes completely distinct from the ectoderm and entoderm, although a jelly-like sustentacular substance may be formed to a greater or less extent between the two primary layers, and cells may pass into it from one of them, so that a kind of mesoderm is thus produced. In the Medusæ, also, the muscular function is performed by 