. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 43 periosteum, the connective tissue of which is directly transformed into bony substance. 3. Muscular tissue. We ascribe the property of contractility to the protoplasm itself of the active cell; but we observe that, even in the protoplasmic body substance of the Infusoria, a striated arrange- ment obtains in those parts in which the contractile function especially resides. By a similar differentiation of the protoplasm certain cells and
. Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote. ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 43 periosteum, the connective tissue of which is directly transformed into bony substance. 3. Muscular tissue. We ascribe the property of contractility to the protoplasm itself of the active cell; but we observe that, even in the protoplasmic body substance of the Infusoria, a striated arrange- ment obtains in those parts in which the contractile function especially resides. By a similar differentiation of the protoplasm certain cells and aggregations of cells possess in a much higher degree the. FIG. 31o.—Myoblasts of a Medusa (Aurelia). power of contractility, and give rise to the so-called muscular tissue which serves exclusively for movement. At the moment of their activity these cells undergo a change of shape; they become shorter and broader than when at rest. In many Coelenterata, cells are found in which a part only of the cell is developed into a contractile fibre. It is the deeper parts of such cells which give rise to delicate muscular fibres or net- works of fibres, while the superficially placed body of the cell * (myoblast), the part which produces the above, performs other functions, and usually bears a ciliurn. In consequence of their epithelial- like arrangement, the myo- blasts receive the name of muscle-epithelium (fig. 34 «, 6). In their further develop- ment the greatest part of the cell protoplasm appears to give rise to contractile muscle- substance ; and sometimes the whole cell becomes elongated into a muscle fibre. Two kind-; of muscles, which are morphologically and physiologically different, are to be distinguished, viz., the smooth muscles, or con- tractile fibre-cells; and the cvoss-striped muscle-substance. '• These cells have been called neuro-muscular cells ; a misleading term, since it cannot be shown that they have had anything to do with the origin of ganglion Ple
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