52 TEXTBOOK OF ZOOLOGY 



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aterial produced by the cells, the cell prmciple soon came to be 

 stated thus : all living things are composed of cells and cell products. 

 With the years, the conceptions of the nature of the nucleus, the 

 cell membranes, and the composition of protoplasm itself have all 

 added their contributions to the present understanding of the mean- 

 ing and application of the cell principle. The cell is now regarded 

 as a physiological unit as well as a structural one, and as almost 

 a corollary to the original statement of the principle, namely, that 

 the activities of the organism equal the sum of the activities of its 

 cells. 



With the embracing of the functional activity of the cell as a part 

 of the principle underlying living processes comes also the inclusion 

 of heredity and development. Cell division, growth, tissue forma- 

 tion, migration of cells, formation of cell products, chromosome rela- 

 tionships and modifications have come to be recognized as being 

 brought about in or by the cells. Through the rather rigid and 

 constant set of developmental changes for which the cells are respon- 

 sible, there is developed a new individual which usually resembles 

 its parents quite closely. 



The influence of the cell theory on biological thinking and progress 

 as well as its effect on fundamental thinking generally, can hardly 

 be over-estimated. The conception of this idea was one of the great 

 landmarks in development of biological ajid scientific thinking. It 

 was the first great generalization in biology. It is comparable in 

 the field of biology to Newton's law of gravitation in the field of 

 physics. Up until this time there had been no single fundamental 

 idea applied to living material that was recognized as being univer- 

 sally true. This conception focused the thinking of all biologists 

 in the same direction and therefore it had a great unifying influ- 

 ence. Deliberation and meditation on this fundamental idea seemed 

 to prepare biologists for other great generalizations which followed 

 quite rapidly. Many new problems arose with this new knowledge 

 of plants and animals. Comparative morphology was extensively 

 investigated, and physiology now has become physiology of cells 

 as a result of this impetus. An understanding of the permeability 

 of cellular membranes, the transformation of energy by chemical 

 reaction within cells, the roles of electrolytes in living substance, 

 and the principles of heredity are some of the results of this new 

 conception of life embraced in the cell theorj^ 



