38 



GENERAL BIOLOGY 



been segregated from the rest, and in this area, 

 one primary function of living substance has been 

 emphasized to the practical exclusion of the others. 

 If a cell-wall should form across the base of the 

 vorticella-bell, and if both cells should then remain 

 together, we should be justified in speaking of the 



FIG. 15. Types of muscle-cells : A and B, smooth muscle-cells ; C, 

 two fragments of cross-striated muscle fibers (cells) ; at the left above, 

 the end of a fiber. Note the numerous nuclei. (Verworn.) 



contractile member of this two-celled organism as a 

 muscle-cell. We know of no instance of such a 

 development in Vorticella or any other protozoan, 

 but in the Metazoa the segregation of the contractile 

 function of protoplasm into a special area and the 

 differentiation of this area into contractile muscle- 

 cells is almost universal. Such a cell is in turn 

 differentiated with respect to the nature of the con- 



