12 



: j $T R I T I Ols A L "Pi YSIQLOG Y 



than had been supposed. Physiologists are bound to be 

 modest in their claims for progress. They are ignorant 

 of many factors at work in even the simplest forms of 

 plant and animal life. And the mystery of consciousness 

 with its relation to nervous systems seems ever to defy 

 approach. 



Free-living Cells. About seventy years ago, at a time 

 when investigators were profiting by important im- 

 provements in microscopes, it was found that the larger 



Fig. 1. Four types of free-living animal cells: A is the ameba, 

 distinguished for its changeable form; B, the euglena, shows the 

 peculiar feature known as a flagellum, a writhing filament, which 

 is its means of locomotion; C is the paramcecium, or " slipper ani- 

 malcule," which has a ciliated surface; D is the interesting form 

 known as the stentor. 



plants and animals are made up of structural units as- 

 sembled in vast numbers. These units are generally 

 much too small to be seen without magnification. They 

 are called cells, a term which is not especially appropriate, 

 but not likely to be abandoned. Many microscopic 

 forms, such as the swarming Infusoria of pools and ditches, 

 are cells leading an independent existence. It will be 

 helpful to consider what are the characteristic activities 

 of such cells. They are for the most part equally charac- 

 teristic of the higher forms. 



