280 



BIOLOGICAL LECTURES. 



have different optical properties from the connecting filamen- 

 tous portion. When contraction sets in, the varicosities on 

 both sides of the intermediate zone (Z, Fig. 7) increase in bulk 



III 



Fig. 7. (I) A schematic representation of the muscle cell. N, nucleus ; s, sarcoplasm ; m , 

 myoplasm ; M, microsome of the filament ; Z, Zwischenscheibe, or Krause's membrane. 



(II) The same in a state of contraction. C, the contraction band. 



(III) Diagram showing the possible mode of formation of the centrosome by the convergence of 

 the fibrils (/) into one common focus. N, nucleus ; C, centrosome. 



at the expense of intervening filamentous substance, and at the 

 maximum state of contraction a thick, new zone is formed as 

 the result of such a process, giving rise to what is known as 

 the contraction band (C, Fig. 7, II). This contraction band 

 has the same chemical and physical property as the ordinary 

 thickenings or varicosities of the fibrils, only much more mas- 

 sive and conspicuous, being formed by the fusion of several 

 varicosities on both sides of the intermediate zone (Z). 



When the relaxation of the muscle sets in, this contraction 

 band resolves itself into a series of smaller varicosities distrib- 

 uted along the fibrils (Fig. 7, I). 



The contraction of the muscle cell means, therefore, the 

 formation of more stainable substance at the expense of less 

 stainable protoplasmic filaments; and the expansion of the 

 muscle means just the reverse of this process, viz., the conver- 

 sion of the deeply stainable varicosities into the less stainable 

 filamentous substance. 



Now, coming back to our original subject, the aster, we 

 notice that, so far as we can judge by the use of staining re- 

 agents, the varicosities in the muscle fibrils and those in the 



