THE MICROSCOPE IN MEDICINE AND SANITATION. 105 



built up of minute individual masses called cells. Quickly 

 the essential identity of the substance forming the cells 

 was postulated. The microscope thus grounded anatomy 

 upon the firm basis of the cell theory; and to-day cytology, 

 the study of the minuter structure of the cell itself, forms 

 an independent subject of ever-increasing importance. 

 Next it was discovered that the cell is not only the unit 

 of adult structure, but the form in which every living 

 organism originates from its parent. In the study of 

 the course of development by which the single cell is 

 transformed into the mature organism, carried out by 

 Kolliker, Hertwig, Van Beneden, and their compeers, 

 embryology came into being. Upon the anatomical and 

 embryological unity thus demonstrated under the micro- 

 scope, the doctrine of evolution, the most important 

 scientific contribution of the nineteenth century, was 

 largely founded. 



2. The Microscope in Medicine and Sanitary Science. — 

 Of all the branches of biology, none owes more to the 

 compound microscope than the study of disease. The 

 conception of the body as a complex of protoplasmic 

 cells, whose normal cooperation was the condition for 

 good health, led easily to the conclusion that disease 

 might arise from the deranged functioning of the in- 

 dividual cell. Cellular pathology, associated with the 

 name of Virchow, led along this Hne to great advances 

 in the knowledge of those diseased conditions which 

 arise from the abnormal activity of the living proto- 

 plasm. 



Another group of maladies due to the invasion of 



