86 



GENERAL PRINCIPLES OF ZOOLOGY. 



the histological change is the formation of specifically 

 functioning plasmic products. If we take as an exam- 

 a ^ pie the manner in which a cell becomes a 



muscle fibre (Fig. 23), we see that it con- 

 tinually secretes upon its surface new fibres 

 of specific muscle substance, in the case of 

 the vertebrates, new cross-striated muscle 

 fibrils, until finally the remnant of the 

 formative cell, the muscle corpuscle, is 

 contained in a mantle of muscle fibrils. 

 In an analogous way, each tissue shows 

 itself upon histological examination to 

 be composed of cells and plasmic prod- 

 ucts. The former care for the formation, 

 FIG. 23. Formation of the renewal, and the sustenance of the 



muscle fibrils in the . , i i , i r , i 



frog. (Diagram.) a, tissue ; the latter are the agents of its phy- 



formative cell; b, . . i r . TM i r 



formative cell with siological function. I he advantages of 



two transversely . r . r , . . 



striated muscle fibrils ; tissue formation are far-reaching, since in 



/. formative cell with . . ........ 



numerous muscle fi- general they are connected with division 

 of labor (frequently referred to later). So 

 long as the cells unite in themselves all the vital functions, 

 they are incomplete because they mutually hinder each 

 other in their free development ; the plasmic product, on 

 the other hand, serves only the single function peculiar 

 to it and can therefore discharge its duties with greater 

 completeness. The muscle fibrils, the characteristic ele- 

 ments of the muscles formed by the cells, have of the 

 various peculiarities of protoplasm preserved only the capa- 

 bility of contraction; but this power of contraction is much 

 more energetic and stronger than the mere movement of 

 protoplasm. The nerve fibrils serve only for the trans- 

 mission of stimuli, but in an extraordinarily more rapid 

 and orderly manner than does simple protoplasm. 



The case is the same as in human life : a man who 

 wishes to be his own tailor, shoemaker, etc., at the same 

 time to study science and art, will with the same talent 

 accomplish less in each branch than another who concen- 

 trates his whole energy in a definite direction. 



