200 



PHYSIOLOGY 



The development of this regular cross and longitudinal striation 

 is closely connected with the evolution and specialisation of the 

 muscular function, i.e. contraction. Contractility is among others a 

 function of all undifferentiated protoplasm. Undiflerentiated cells, 

 such as the amceba, can effect only slow and weak contractions. 



FIG. 36 



FIG. 35 



B 



Fid. 35. Muscle fibre of an ascaris. a, the differentiated contractile 



portion of the cell. (After HERTWIG.) 

 FlQ. 36. Muscle fibres from the small intestine, showing the fine longitudinal 



striation. (SCHAFER.) 



Directly a specialisation of function is necessary and some cell or 

 part of a cell has to contract rapidly in response to some stimulus 

 from within or without, we find a differentiation both of form and of 

 internal structure. In many cases, as in the developing muscle of 

 the embryo or the adult muscles of many invertebrates, this differentia- 

 tion affects only part of the cell, so that while one part presents the 

 ordinary granular appearance, the other half is finely and longitu- 



