. Elements of comparative anatomy. Anatomy, Comparative. 30 COMPARATIVE ANATOMY. b. The Animal Tissues. § 25. In the epithelial, as well as in connective tissues, the product of the differentiation of the protoplasm gives rise to phenomena which are limited to the sphere of vegetative operations. When a more highly contractile substance arises as a product of the division of the protoplasm, a new tissue is formed, which is known as contractile or muscular tissue. Its contractility, however, is not automatic, but dependent on stimuli, which come from the form- elements of the nervous system. Th
. Elements of comparative anatomy. Anatomy, Comparative. 30 COMPARATIVE ANATOMY. b. The Animal Tissues. § 25. In the epithelial, as well as in connective tissues, the product of the differentiation of the protoplasm gives rise to phenomena which are limited to the sphere of vegetative operations. When a more highly contractile substance arises as a product of the division of the protoplasm, a new tissue is formed, which is known as contractile or muscular tissue. Its contractility, however, is not automatic, but dependent on stimuli, which come from the form- elements of the nervous system. The contractile form-elements of the muscular tissue differ therefore essentially from the indifferent cell formed of protoplasm, although the latter also is contractile. They presuppose the existence of another, or nervous tissue, just as it on the other hand determines the existence of the muscular tissue. These intimate re- lations explain the causal relationship which these two tissues have to one another phylogenetically. The two kinds of elements are differentiated from a single neuro-muscular cell, which is in many Ccelenterata the representative of the two tissues. (Fig. 13.) This kind of cell corresponds to an indifferent stage of the animal tissues, in which they have not yet be- come distinct tissues. The tissue which forms the starting-point of the differ- entiation is not a new structure. It is the outermost layer of the body, and consists of cells, which form an epithe- Fig. 13. Neuro-muscular cells iium> The neuro-muscular tissue of Hydra. n Processes of the • ,i p -i-™. ,. ,. P ,, cells, m Contractile fibres (after 1S therefore a differentiation from the Kleineuberg). epithelial tissue, and is thus connected with a more simple condition. Cells which are hardly at all different from other epithelial cells give off a band-like process at their base, which becomes connected to a layer of longitudinal fibres underlying the epithelium. While the epithelial cell
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