ORGANIZATION AND DEVELOPMENT OP 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 ; bat 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 

 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 FIG. 3J. Myobiasts of a Medusa 



a change of shape,; they become 

 shorter and broader than when at rest. 



In many Ccelenterata, 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, 

 pei-forms other functions, and 

 usually bears a cilium. In 

 consequence of their epithelial- 

 like arrangement, the myo- 

 blasts receive the name of 

 muscle-epithelium (fig. 34 , 

 f>}. 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 kinds of muscles, which are morphologically and physiologically 

 different, are to be distinguished, viz., the smooth muscles, or con- 

 tractile fibre-cells ; and the cross-striped muscle-substance. 



'' These cells have been called neuro-musrular cells ; a misleading term, since 

 it cannot be shown that they have had anything to do with the origin of 

 ganglion cells. 



FIG. 344. Muscle-epithelium of a Medusa 

 ( Anrelia) . 



