GENERAL ZOOLOGY 



that cork is composed of small spaces surrounded by firm walls, and in 1665 

 he named these compartments cells. The development of microscopic instru- 

 ments was very slow, and it was not until 1831 that Robert Brown observed 

 in certain plant structures that each cell contains a small body, which he 

 called the nucleus. In 1838 Schleiden, a German botanist, proposed the in- 

 terpretation that cells are the units of structure in plants; and Schwann, a 

 German zoologist, extended this conception to the structure of animals in 

 1839 (Fig. 2.1). This was the first formulation of the Cell Theory. The 

 founders of this theory and other biologists of that time believed that the walls 

 surrounding plant cells were the essential parts of these units. The contents 

 of cells had been observed but were regarded as unimportant or as waste 

 products. Purkinje in 1840 and von Mohl in 1846 gave the name protoplasm 

 to the cell contents. Through a series of researches it became apparent that 

 protoplasm was the essential part of cells; walls were found only around plant 

 cells. Likewise, a nucleus was found to be present in almost all types of cells, 

 and a cell came to be defined as a mass of protoplasm containing a nucleus. 

 As knowledge increased, it was ascertained that in certain parts of the 

 animal much non-cellular material lies between cells. This material, examples 

 of which will be given later, was shown to be produced by cells, and the Cell 

 Theory was modified to state that organisms are composed of structural units, 

 called cells, and of cell products. Further study of animals with reference to 

 their activities has revealed the fact that all physiological processes must be 

 understood in terms of the functions of cells; that is, the cell is the unit of 

 function. The Cell Theory has, therefore, been extended and confirmed, and 

 it now stands as one of the fundamental generalizations of biological science. 

 A complete statement of this unifying conception of biology would be that 

 all living organisms are composed of cells, the units of structure and function, 

 and of cell products. In complex organisms these units are not independent 

 but are coordinated by their own activities, so that the animal as a whole 

 may also be regarded as a unit. This obvious fact of interrelation among 

 the cells of an individual is sometimes emphasized in what is called the 

 Organismol Theory. In the final analysis the organism is regarded as a proto- 

 plasmic unit according to this theory, and there is no essential difference 

 between it and the modern interpretation of the Cell Theory, which is much 

 easier to use in understanding the structure and function of an animal. 



Distinguishing Capacities of Living Organisms 



Living organisms differ from lifeless things in three fundamental respects. 

 These distinguishing capacities are metabolism, responsiveness, and 

 reproduction. 



Metabolism is the capacity as a result of which cellular constituents are built 

 up and destroyed in such a way that the organism lives. The chemical re- 

 actions occurring during metabolism can be grouped under the headings 



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