120 PHARMACEUTICAL BACTERIOLOGY 



tained to cork tissue. Nothing further of moment was done until about 

 1833 when Robert Brown again took up the study of the cellular elements 

 of plants. Thus it will be seen that a period of nearly two hundred years 

 had elapsed since the first observations on plant cells during which no 

 notable progress was made in the study of cytology. This long period of 

 inactivity was no doubt occasioned by the inadequacy of the simple 

 microscope and the fact that the compound microscope was not perfected 

 sufficiently to be of any great advantage over the simple microscope, until 

 about 1825. By 1838 and 1839 Schleiden and Schwann were ready to 

 formulate a cell theory of living things, based largely upon the study of 

 plant tissues and organs. At this time it was generally believed that the 

 cell-wall represented the basic or essential part of the cell. In 1846 von 

 Mohl called attention to a slimy or mucilaginous substance enclosed by 

 the cell-wall which he called protoplasm, believing it to be the primal 

 living substance. Dujardin, Cohn and others, now began to give some 

 attention to the animal structure, declaring that this also consisted of 

 cellular elements in which were often noticeable certain slimy or gelatinous 

 substances to which Dujardin gave the name sarcode. Kb'lliker noted the 

 fact that the animal cell was frequently devoid of a cell-wall. Max 

 Schultze declared that the protoplasm of the plant cell and the sarcode of 

 the animal cell were in all essentials alike. Since 1870 a multitude of 

 keen observers and able investigators have given their attention to the 

 plant cell and the animal cell and some of the theories based upon the 

 discoveries made have become classics in scientific annals. To review 

 these is impracticable and unnecessary for the present purpose. Atten- 

 tion may however be called to the more important investigations and 

 theories. 



It may be recalled that Harvey in the i6th century declared that all 

 life came from an egg (Omne vivum ex ovo) which, after the formulation of 

 the cell theory, was changed to the declaration that every cell came from 

 a preexisting cell (Omnis cellula ex cellula), and, no life excepting from a 

 previously existing living organism (Omne vivum e vivo). The cell- wall 

 lost interest and the entire attention was now centered upon the cell- 

 contents. For a time an attempt was made to differentiate between 

 living and non-living or dead cell contents, but soon the conclusion was 

 reached that all cell parts and cell constituents were the product of plasmic 

 activity at some time since their coming into existence. Nor did it take 

 long to reach the conclusion that even the cell was not the ultimate unit 

 of living structure, that it was rather a living complex of which we know 

 very little and of which we cannot know very much until the chief me- 

 chanical aid to biologic investigation, namely the compound microscope, is 

 more highly perfected and until the science of physical and biologic chemis- 



