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 cells. 



FIG. 34i. Muscle-epitlieliuin of a Medusa 

 (Aurelia). 



