30 



COMPARATIVE ANATOMY. 



B. The Animal Tissues. 

 § 25. 



In the epithelial, as well as in connective tissuesj the product 

 of the differentiation of the protoplasm gives rise to phsenomena 

 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 Coelenterata 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- 

 lium. The neuro-muscular tissue 

 is therefore a differentiation fi-om the 

 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 cells of the outer layer of the body unite, when in 

 their indifferent condition, a low grade of sensibility with a low 

 grade of contractility, the sensibility, when the cells become more 

 highly specialised, remains with them, and the contractility becomes 

 assigned to a differentiated process of the protoplasm, which now 

 appears as a distinct appendage of the cell. Thus commences 

 that arrangement which, in the more highly differen- 

 tiated stages, is expressed by the connection between 

 ganglion-cells, nerve-fibres, and muscle-fibres. By sup- 

 posing that the fibres, which in the earlier case appear to be 

 merely processes of the cells, retain their nuclei, and that the 

 products of the division of the nuclei of the cell gradually become 

 fibres, and that further the neuro-muscular cell is no longer con- 



Fig. 13. Neuro-muBcular cells 



of Hydra. n Processes of the 



cells, m Contractile fibres (after 



Kleiaenberg) . 



