SEPTEMBEE7, 1900.] 



SCIENCE. 



361 



bladed knife invented by Valentin, held 

 in the hand, was the only improvement 

 on the scapel or razor for cutting thin, 

 more or less translucent slices suitable for 

 microscopic examination ; mechanical sec- 

 tion cutters and freezing arrangements had 

 not been devised. The tools at the disposal 

 of the microscopist were little more than 

 knife, forceps, scissors, needles ; with acetic 

 acid, glycerine and Canada balsam as re- 

 agents. But in the employment of the 

 newer methods of research care has to be 

 taken, more especially when hardening and 

 staining reagents are used, to discriminate 

 between appearances which are to be inter- 

 preted as indicating natural characters, and 

 those which are only artificial productions. 

 ^Notwithstanding the difficulties attend- 

 ant on the study of the more delicate tis- 

 sues, the compound achromatic microscope 

 provided anatomists with an instrument of 

 great penetrative power. Between the years 

 1830 and 1850 a number of acute observers 

 applied themselves with much energy and 

 enthusiasm to the examination of the min- 

 ute structure of the tissues and organs in 

 plants and animals. 



CELL THEOET. 



It had, indeed, long been recognised that 

 the tissues of plants were to a large extent 

 composed of minute vesicular bodies, techni- 

 cally called cells (Hooke, Malpighi, Grew). 

 In 1831 the discovery was made by the 

 great botanist, Kobert Brown, that in many 

 families of plants a circular spot, which he 

 named areola or nucleus, was present in 

 each cell ; and in 1838 M. J. Schleiden pub- 

 lished the fact that a similar spot or nucleus 

 was a universal elementary organ in vege- 

 tables. In the tissues of animals also 

 structures had begun to be recognized com- 

 parable with the cells and nuclei of the 

 vegetable tissues, and in 1839 Theodore 

 Schwann announced the important general- 

 ization that there is one universal princi- 



ple 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 principle that the elementary 

 tissues consisted of cells constituted a step 

 in the progress of biological science, which 

 will forever stamp the century now draw- 

 ing 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 

 generated 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 physi- 

 ologist and pathologist with the specific 

 structures through the agency of which the 

 functions of organisms are discharged in 

 health and disease. It exerted an enormous 

 influence on the progress of practical medi- 

 cine. A review of the progress of knowledge 

 of the cell may appropriately enter into an 

 address on this occasion. 



STEUCTUEE 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 stim- 

 uli, 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 dis- 

 charged. The original conception of a cell, 

 based upon the study of the vegetable tis- 

 sues, was a minute vesicle enclosed by a 

 definite wall, which exercised chemical or 

 metabolic changes on the surrounding ma- 

 terial and secreted into the vesicle its char- 

 acteristic contents. A similar conception 

 was at first also entertained regarding the 

 cells of animal tissues ; but as observations 



