GENERAL BIOLO&Y. 27 



body ; nourishment passes from cell to cell, and waste is dis- 

 charged into the water surrounding the creature from all cells, 

 though probably not quite equally. All parts are not digestive, 

 respiratory, etc., to the same degree, and herein does it differ 

 greatly from Amoeba or even Vorticella, though fuller knowl- 

 edge will likely modify our views of the latter two and similar 

 organisms in this regard. 



THE CELL RECONSIDERED. 



Having now studied certain one-celled plants and animals, 

 and some very simple combinations of cells (molds, etc.), it will 

 be profitable to endeavor to generalize the lessons these humble 

 organisms convey ; for, as will be constantly seen in the study of 

 the higher forms of life of which this work proposes to treat 

 principally, the same laws operate as iu the lowliest living creat- 

 ures. The most complex organism is made up of tissues, which 

 are but cells and. their products, as houses are made of bricks, 

 mortar, wood, and a few other materials, however large or elab- 

 orate. 



The student of physiology who proceeds scientifically must 

 endeavor, in investigating the functions of each organ, to learn 

 the exact behavior of each cell as determined by its own inherent 

 tendencies, and modified by the action of neighboring cells. 

 The reason why the function of one organ differs from that of 

 another is that its cells have departed in a special direction from 

 those properties common to all cells, or have become function- 

 ally differentiated. But such a statement has no meaning un- 

 less it be well understood that cells have certain properties in 

 common. This is one of the lessons imparted by the preceding 

 studies which we now review. Briefly stated in language now 

 extensively used in works on biology, the common properties of 

 cells (protoplasm), whether animal or vegetable, whether consti- 

 tuting in themselves entire animals or plants, or forming the 

 elements of tissues, are these : The collective chemical processes 

 associated with the vital activities of cells are termed its metdb- 

 olism. Metabolism is constructive when more complex com- 

 pounds are formed from simple ones, as when the Protococcus- 

 cell builds up its protoplasm out of the simple materials, found in 

 rain-water, which makes up its food. Metabolism is destructive 

 when the reverse process takes place. The results of this process 

 ai-e eliminated as excreta, or useless and harmful products. 



