CHAPTER XII 

 THE CELL 



" Citizens of the state which the entire multicellular organism seems to be." 



WE have seen that foodstuffs are broken down into units suffi- 

 ciently small to pass through the intestinal wall. Logically, 

 we ought next to study the system by which these absorbed 

 substances are conveyed to the cell. It is important to realise 

 that until they are inside the cell they are useless. All energy 

 changes take place in the living cell. It will, however, be convenient 

 first to examine a cell, note its imports and exports and study the 

 various activities by virtue of which it is said to be alive. 



In 1893, Schwann put forward the theory that animal tissues 

 were an aggregation of large numbers of cells. Later work has 

 justified this assumption. It is now generally held by biologists 

 (1) that the earliest form of living matter Avas undifferentiated 

 protoplasm, and that from this simple form of life there has been 

 evolved, first the unicellular and then the polycellular organism ; 

 and (2) that each individual life follows this evolutionary course, 

 originating as a single cell and gradually gaining in complexity 

 with age. In view of these tw r o beliefs, the evolutionary hypo- 

 thesis (phylogeny) and the developmental history (ontogeny), it 

 is logical to subject a unicellular organism to close examination 

 in order that the various manifestations of life may be, at least, 

 catalogued. 



Amoeba is a unicellular animalcule which may be found in the 

 stagnant water of almost any ditch. It is made of material 

 differing so slightly from the medium in which it lives that it can 

 only be seen under the microscope after patient search. When 

 seen it is found to be non-homogeneous. Apparently it consists 

 of a greyish mass in which there are occasional granular aggregates, 

 spaces containing water, spaces containing extraneous matter and 

 a darker more compact mass, the nucleus. If the amoeba were 



B.B. l-2<) 9 



