ADDRESS BEFORE THE BRITISH ASSOCIATION. S^7 



Schwann announced the important generalization that there is one uni- 

 versal principle of development for the elementary part of organisms, 

 however different they may be in appearance, and that this principle 

 is the formation of cells. The enunciation of the fundamental prin- 

 ciple that the elementary tissues consisted of cells constituted a step 

 in the progress of biological science which will forever stamp the cen- 

 tury now drawing to a close with a character and renown equalling those 

 which it has derived from the most brilliant discoveries in the physical 

 sciences. It provided biologists with the visible anatomical units 

 through which the external forces operating on, and the energy gen- 

 erated in, living matter come into play. It dispelled forever the old 

 mystical idea of the influence exercised by vapors or spirits in living 

 organisms. It supplied the physiologist and pathologist with the spe- 

 cific structures through the agency of which the functions of organisms 

 are discharged in health and disease. It exerted an enormous influ- 

 ence on the progress of practical medicine. A review of the progress 

 of knowledge of the cell may appropriately enter into an address on this 

 occasion. 



STRUCTURE OF CELLS. 



A cell is a living particle, so minute that it needs a microscope for 

 its examination; it grows in size, maintains itself in a state of activity, 

 responds to the action of stimuli, reproduces its kind and in the course 

 of time it degenerates and dies. 



Let us glance at the structure of a cell to determine its constituent 

 parts and the role which each plays in the function to be discharged. 

 The original conception of a cell, based upon the study of the vegetable 

 tissues, was a minute vesicle inclosed by a definite wall, which exer- 

 cised chemical or metabolic changes on the surrounding material and 

 secreted into the vesicle its characteristic contents. A similar concep- 

 tion was at first also entertained regarding the cells of animal tissues; 

 but as observations multiplied, it was seen that numerous elementary 

 particles, which were obviously in their nature cells, did not possess an 

 inclosing envelope. A wall ceased to have a primary value as a con- 

 stituent part of a cell, the necessary vesicular character of which there- 

 fore could no longer be entertained. 



The other constituent parts of a cell are the cell plasm, which forms 

 the body of the cell, and the nucleus embedded in its substance. Not- 

 withstanding the very minute size of the nucleus, which even in the 

 largest cells is not more than one-five-hundredth of an inch in diameter, 

 and usually is considerably smaller, its almost constant form, its well- 

 defined sharp outline and its power of resisting the action of strong 

 reagents when applied to the cell, have from the period of its discov- 

 ery by Robert Brown caused histologists to bestow on it much attention, 



