Lefevre — The Advance of Zoology in the Nineteenth Century. 95 



almost within the past twenty-five years. This remarkable 

 growth which has taken place in our knowledge of the struc- 

 ture and activities of cells has been immensely aided, in fact 

 made possible, by the great improvement within recent years 

 of microscopical lenses, by the invention of accurate micro- 

 tomes and by the perfection of methods of hardening, stain- 

 ing, imbedding and serial section-cutting. It has thus become 

 possible in cytological research to preserve, with little distor- 

 tion, the most delicate of cell-structures, to bring into view, 

 by means of differentiating stains, objects which would other- 

 wise be invisible, and to examine them, in sections of only 

 one thousandth of a millimeter in thickness if need be, under 

 remarkably high powers of magnification. 



In all the higher forms of animal and plant life the body 

 consists of innumerable structural units, termed cells, out of 

 which, directly or indirectly, every part is constructed; and 

 the view that all organisms are composed of these elementary 

 minute particles is known as the cell-theory which is rightly 

 considered to be one of the most important generalizations in 

 the history of modern biology. 



The essential substance composing cells is living matter or 

 protoplasm which was termed by Huxley the '* physical basis 

 of life " and which is now universally regarded as the seat 

 of all manifestations of life. In the lowest organisms the 

 body consists of a single cell in which all of the vital func- 

 tions are performed ; in the higher forms, however, the body 

 is made up of a multitude of cells and is in a certain sense to 

 be compared with a colony or aggregate of many unicellular 

 forms which exhibit a division of labor among themselves, 

 some being specially modified in structure for the perform- 

 ance of one function, others modified in a different direction 

 for another function. And as the functions of the organism 

 as a whole are but the result of the activities of the individual 

 cells, we therefore recognize the cell, not only as the unit of 

 structure, but as the unit of function as well. *< Considera- 

 tion of the individual functions of the body urges us con- 

 stantly toward the cell. The problem of the motion of the 

 heart and of muscular contraction resides in the muscle-cell ; 

 that of secretion in the gland-cell ; that of food-reception and 



