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 aspect, some another, of their vital attributes. Thus one cell specializes in, say, secretion, another in contractility, another in receiving and carrying stimuli, and so forth, so that we have the gland cell, the muscle cell, and the nerve cell, each appropriately grouped with its fellows to constitute the particular tissue or organ—gland, muscle or brain—which has for its function that of its constituent cells. In unicellular animals we also find division of labour and its accompanying morphological differentiation, but here there is no subdivision of the protoplasm of the organism into the semi-autonomous units which so greatly facilitate division of labour in the Metazoa; instead, division of labour must be between different regions of protoplasm in the single cell. The sharply defined character of this regional differentiation in the Protozoa, and the surprising structural complexity it may produce, sufficiently clearly show that although multicellular structure has greatly facilitated regional differentiation in the Metazoa, it is by no means essential to this process (see below, Present Position of the Cell-theory).

It is not within the scope of this article to attempt a comprehensive review of the variety in structural complexity to which this division of labour among the cells of the Metazoan and the regional differentiation of the cell-bodies of the Protozoa has given rise. Some indication of the wealth of variety may be best given by taking a general survey of cell-modifications, grouped according to the cell-attributes the expression of which they facilitate.

a and b from Schäfer’s Essentials of Histology, by permission of Longmans, Green & Co.

—Types of Cells. a, Fat-cell enclosing a huge fat-globule. b, Part of a Mammalian “striated” muscle-cell (diagrammatic). c, Spermatozoa of mouse and bird. (a) Structural Complexity facilitating Movement.—One of the most striking, and hence earliest described, of the fundamental attributes of protoplasm is its power of spontaneous movement. This is seen in the walled cell of plant tissue and in the naked cell-body of Amoeba. In the latter case the streaming movements of the naked protoplasm are accompanied by the formation of “pseudopodia,” and result in the highly characteristic “amoeboid” creeping movement of this and similar organisms (e.g. lymph corpuscles of the blood). In these examples the whole protoplasm participates in the movement,—there has been no division of labour, and there is, therefore, no visible morphological differentiation. In many cells, movement (either of the entire body or of the surrounding medium) is by means of slender whip-like processes of the protoplasm flagella or cilia. These represent modified pseudopodia, and in the formation of the motile gametes of some of the lower forms, e.g. Myxomycetes (de Bary, 1859), Rhizopods (R. Hertwig, 1874), &c., the actual conversion of a pseudopodium into a flagellum can be witnessed. These vibratile processes may be either one or few in number, and are then large in size and move independently of one another; or they may be very numerous, covering the free surface of the cell (fig. 2, a); they are then very small and move strictly in unison. In the former case they are termed “flagella,” in the latter “cilia.” In some cases the flagellum is accompanied by an undulating membrane (e.g. Trypanosoma among the protozoa and in many spermatozoa), and it may be situated either at the front end (Euglena) or hind end (spermatozoa) of the body during motion. The cilia may form a uniform coating to the free surface of the cell, as in ciliated epithelium (fig. 2, a) and many infusoria, or the cilia may be variously modified and restricted to special regions of the body, e.g. the “undulating membrane” of the peristomial region in many infusoria, the swimming combs of the (q.v.), and the flame cells of the (q.v.). In one group of infusoria (Hypotricha), the cilia, “cirri,” have attained a high degree of differentiation, and reach a considerable size. Both cilia and flagella spring directly from the cell-protoplasm, piercing the cell-membrane, when this is present. At the point where they become continuous with the cell-body there is usually a deeply staining “basal granule.” In some cases the flagella are in direct connexion with the centrosome (see below, Cell-division), e.g. Trypanosoma and spermatozoa, in some cases even while the centrosome is functioning in mitosis (e.g. insect spermatogenesis, Henneguy and Meves (fig. 3).

From A. Gurwitsch, Morphologie und Biologie der Zelle, by permission of Gustav Fischer.

—Types of Cells. a, Ciliated epithelial cells. (After Heidenhain.) b, Mucus-secreting “goblet”-cells. (After Gurwitsch.)



From O. Hertwig, Allgemeine Biologie, by permission of Gustav Fischer.

—Spermatocytes of Bombyx mori, showing the precocious appearance of the spermatozoon flagellum and its relation to the centrosome. (After Henneguy.) In the ability of Amoeba to contract into a spherical mass, and in the presence in its protoplasm of the contractile vacuole, we see another type of spontaneous movement—contractility—of the protoplasm. In the “musculo-epithelial” cells of Hydra, the elongated basal portion of the cell alone possesses this contractility. In the higher Metazoa the whole cell—muscle cell—is specialized for contractility, and shows, as a result of its specialization, a distinct fibrillation. This fibrillation is foreshadowed in the contractile regions of many Protozoa, e.g.