18 PHYSIOLOGY. 



increase in the size and weight of the various organs of his body. 

 This means that there is a greater number of cells composing the 

 tissues of his various organs. The power to multiply that is, pro- 

 ducing new forms similar to itself is one of the most important 

 and characteristic functions of the cell. By this attribute, it not 

 only is able to maintain its own particular kind, or species, but can 

 undergo constructive metamorphosis : building up, or growing, until 

 any part, or organ, is matured. 



A cell multiplies by dividing into two or more parts. Each 

 part is, of course, smaller than the original or mother-cell, but, by 

 assimilating nutrient material from the surrounding tissues, it 

 grows until each part is the size of the mother-cell, when it also is 

 ready for division, or reproduction. 



No cell exists that has not had its origin in some pre-existing 

 cell. In animals whose tissues are composed of many cells, these 

 same tissues can be traced back to single cells, of which they are 

 developments. The animal itself, with all its many and various 

 parts and structures, originated from a single cell, the germ-cell, or 

 ovum, which have existed in the parent-body, is also derived from 

 a cell. 



Schleiden, the botanist and accredited discoverer of the cell- 

 theory among plants, and Schwann, to whom Schleiden confided his 

 views and ideas of plant-structure, and who then reduced animal tis- 

 sues to their structural units, the cells, were anxious to know the 

 origin of the cells. To them the presence of the nucleus was known, 

 and even the nucleolus; but their instruments were not powerful 

 enough to allow of their penetrating deeper, and of getting the cor- 

 rect ideas of cell-division. 



It was proved in 1858 that cells multiplied as a result of the divi- 

 sion of the two equally essential parts of the cell, the nucleus and 

 protoplasm. Our present conception, that the two are of equal im- 

 portance and value, dates from this time. It was asserted that the 

 division began within and proceeded to the outer parts of the cell. 

 That is, the nucleolus was divided, its division was followed by sepa- 

 ration of the nucleus, and this, in turn, followed by constriction and 

 division of the protoplasm with its enveloping membrane. These 

 views were confirmed by Virchow, who formulated the doctrine 

 "Omnis cellula e cellula" (every cell from a cell). 



Later, it was discovered, by the investigation of some of the 

 tissue-cells, that the process of division was not so simple as expected. 

 In some cases, it was found that the nucleus became star-shaped, or 



