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 in the cirri of hypotrichous Infusoria, the tentacle of Noctiluca, and the myophane layer of Gregarines. In the quickly contracting muscle cell of Vertebrates and insects, further specialization has produced a structure of considerable complexity (fig. 1, b). Here also the cell is fibrillated, but the fibrillae (sarcostyles) are much more distinct, and are segmented in a manner which gives to the entire cell a “cross striated” appearance. Since quick movement is usually (but not always) associated with voluntary control, these striated muscle cells are often termed “voluntary” muscle fibres. The great increase in length of these cells is accompanied by the fragmentation of the originally single nucleus.

(b) Cell-modification in Relation to Secretion.—Just as the complex movements considered above were the result of a great development of the power of spontaneous movement possessed by all protoplasm, so cell-secretion is the result of a development of the metabolic processes underlying all vital phenomena. But whereas specialization of the protoplasm for movement resulted in a very obvious morphological complexity, specialization for secretion results in molecular complexity, and only rarely and indirectly results in morphological differentiation. Usually indeed the specialization is only rendered evident by the appearance of the formed secretion, e.g. mucus-secreting epithelial cells (fig. 2, b), the ovarian ovum and the fat cell (fig. 1, a). In some cases a distinct fibrillation of the cytoplasm accompanies or precedes the appearance of the cell-secretion (Mathews, pancreas cell of Amphibia). In many cases the internal secretion is no mere accumulation, e.g. the internal skeleton of the Radiolaria, and the nematocysts of the Coelentera. Frequently in animal tissues the cell-secretions are accumulated in the intercellular spaces, and result in the formation of the various “connective tissues,” all of which are characterized by the immense amount of intercellular substance, e.g. fibrous tissue, cartilage and bone. Cell-modifications facilitating the general metabolism, but not necessarily indicating specialized secretion, also occur, e.g. the “gullet” of many Protozoa, the suctorial tubules of the Acinetaria, and the “nutritive processes” of the ovarian ova in many Lepidoptera. Mention may be made here of the network or canal system of the cytoplasm, described for many cells by Golgi, Holgren and others. An enigmatical structure, the “yolk-nucleus” of many ova, has been frequently regarded as a structure of considerable metabolic importance, e.g. Bambeke (1898) for Pholcus.

From Prof. E. B. Wilson’s The Cell in Development and Inheritance, by permission of the author and of the Macmillan Co., New York.

a, Permanent spireme-nuclei in cells from the intestinal epithelium of a dipterous larva, Ptychoptera. (After van Gehuchten.)

From Korschelt and Heider, ''Lehrbuch der verg. Entwicklungsgeschichte der'' wirbellosen Tiere, by permission of Gustav Fischer.

b, Branched nucleus of the “nutritive” cell, from a portion of an ovarial tube of Forficula auricularia. Striking modifications resulting from specialization in secretion are frequently presented by the nucleus. In many secreting cells this structure is extensively branched, e.g. many gland cells and ovarian nutritive cells of insects (fig. 4, b). In some cases the nucleus of the gland cell contains a persistent spireme thread (fig. 4, a); while almost all actively secreting cells are characterized by the possession of large or numerous nucleoli.

From Schäfer’s Essentials of Histology, by permission of Longmans, Green & Co.

A and B, Ganglion cells from the cerebral cortex; in A the only slightly branched axon may extend the whole length of the spinal cord. (After Schäfer.)

C, Body of a ganglion-cell showing “Nissl’s granules.”

D, Sensory cells from olfactory epithelium. (After Schultze.)

E, Diagrammatic representation of the sensory epithelium of retina (rod and cone layer). (After Schwalbe.) (c) Specialization for the Reception and Conduction of Stimuli.—One of the most striking of the fundamental attributes of living protoplasm is its “irritability,” that is to say, its power of responding to external impressions, “stimuli,” by movement, which, both in kind and intensity, is wholly independent of the amount of energy expended by the stimulus. The stimulus conveyed by the nerve fibre to the muscle is out of all proportion to the amount of work it may cause the muscle to do. Although protoplasmic irritability is thus incapable of a simple mechanical explanation, science has rejected the assumption of a special “vital force,” and interprets protoplasmic response as being a long series of chemico-physical changes, initiated, but only initiated, by the original stimulus; the latter thus standing in the same relation to the response it produces as the pull on the trigger to the propulsion of the rifle bullet. The function of receiving stimuli from the outer world, originally possessed to a greater or less extent by all cells, has, in the Metazoa, been relegated to one class of cells, the sensory cells (fig. 5, D and E). Another class of cells—the “ganglion cells” or “neurones” (fig. 5, A and B), are concerned with the conduction of the stimuli so received. The contractile elements in the Metazoa are thus dependent for their stimuli on the nervous elements—the sensory cells and neurones.

Origin of Cells.—In the preceding sections we have considered the structure of the cell in relation to the fundamental attributes of cell-metabolism, irritability, and movement. We have now