KINETOGENESIS. 253 



Arthropoda, and some Vertebrata (Batra- 

 chia). And when the entire muscle-fiber is 

 divided into fibrillae, there can appear an 

 external layer of muscular threads. 



"(4) That muscle cells and fibers first appear in 

 the external stratum of the active body of animals, is 

 due to the especially active movements necessary to 

 this part of the body. This is a simple mechanical 

 consequence of the fact, that in a more or less longi- 

 tudinally extended body of protoplasm, whether it be 

 Protozoon or worm, or muscle-cell or muscle-fiber, that 

 its movements must be more vigorous on the external 

 than the internal portion of it. Therefore, the former 

 would first display muscular structure. 



" (5) If the first stage is the development of masses 

 of plasma, which display contraction in definite direc- 

 tions, the next step is the division of such masses into 

 muscle-threads or fibrillae. These threads must be re- 

 garded as a result of contractions, whose inequality 

 produces subdivisions of the original mass. A com- 

 pound structure is also more effective than a simple 

 one in effecting contractions. 



"(6) The next stage of evolution of muscular tissue 

 consists of the appearance of the cross-striping. The 

 mechanical effect of the cross-striping is to distribute 

 the contractility equally throughout the length of the 

 fiber. The contractions of the unstriped muscular 

 fiber are less vigorous, and also less uniformly dis- 

 tributed than those of the striped fiber. The cross- 

 striping is a result of contractions. It commences as 

 simple undulations of the surface of the fiber. The 

 contraction of the plasma is wave-like and is propa- 

 gated rapidly through the fiber, and is not due to a 

 flow and return of the contained protoplasm. The 



