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 thé 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 coéperation 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 line 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 
