32 FUNDAMENTAL PRINCIPLES 



transmitted, and therefore it determines the character of the cell. The 

 central body with its centriole, or centrioles, is active in cell division. 

 Plastids are living structures with active functions and are more numerous 

 in plant than in animal cells; the chlorophyll bodies, which are one form of 

 plastids, utilizing the energy of the sun's rays, build up carbohydrates 

 from carbon dioxide and water. As indicated in the preceding topic some 

 of the other structures play only a passive role, while the functions of 

 others are not definitely known. 



43. Development of Knowledge of the Cell. — The history of this 

 development may be briefly summarized as follows: Hooke, an English 

 microscopist, discovered in 1665 that cork was divided into little com- 

 partments which, because they reminded him of the cells in a monastery, 

 he called cells. In 1833, or 168 years later, Brown, also an Englishman, 

 discovered the nucleus, and it was then supposed that the cell consisted 

 of a living wall inclosing a nonliving, watery substance in which floated 

 the nucleus, also living. It was not until 1835 that Dujardin, a French- 

 man, as has already been stated (Sec. 27), discovered that this watery 

 content of the cell was a substance of peculiar character and that it, too, 

 was living. From this time the cell was believed to contain these three 

 elements, which were found to be common to both plants and animals. 

 It was discovered after a time, however, that cells existed which did not 

 possess a cell wall. Thus the wall, which was at first supposed to be the 

 essential part of the cell, was finally eliminated as a part of it and the word 

 cell became really a misnomer. The most important contribution to the 

 modern conception of the cell was that of Max Schultze, who, in 1861, 

 showed that the substance of all cells, plant and animal, was similar, and 

 who defined a cell as a "small mass of protoplasm endowed with the 

 attributes of life. " 



44. Cell Theory and Cell Doctrine. — The cell theory was due to the 

 work of Schleiden, a botanist (1838), and of Schwann, a zoologist (1839). 

 Each of these men had found cells in all living matter which he had 

 studied, and they presented, each in a publication in his own field, a 

 hypothesis which has been known as the cell theory, to the effect that hving 

 matter always exists in the form of cells. It was to them a theory, but in 

 the time that has elapsed since the dates mentioned it has been found to 

 hold good for all living substance which has been studied. Thus today we 

 no longer consider it a theory but rather a fact, and so it has come to be 

 known as the cell doctrine. This conception when first presented had a 

 most profound effect upon biological thought, and its influence has been 

 equaled only by that exerted by Darwin's theory of evolution. 



