between the hemispheres of the brain is a little sac about the size of a pea, the pineal gland, and comparison shows that this was once a third eye. Sometimes an opening persists on the side of the neck below the jaw; in such a case one of the embryonic neck clefts has remained open, and this in turn has relations to the gill slits of a fish. All the ground plan of our bodies, the muscle cylinder within the skin, next the bony scaffolding, innermost the peritoneal sack around the viscera, all such relations would remain a mystery did we study only the human body. But in the light of comparative anatomy and embryology we recognize them as necessary parts of our heritage. Medicine must stand upon a thorough knowledge of the structure and processes of the human body, and before it can treat disorders it must understand states of health and their origin. Comparative anatomists and embryologists, the great men Harvey, Wolff, von Baer, Cuvier, Agassiz, Huxley, Cope and Gegenbaur, such men have not only broadened the field of human thought, but have also furnished the understanding of the human organism. They were all pure scientists, they did not have in mind the care and cure of the human body. Yet we might say they accomplished more for a rational medicine than all the physicians before them. How unlikely the prophecy seemed that any direct advantage would come to mankind from the researches of Harvey, Wolff and von Baer on the development of the chick, from those of Cuvier and Agassiz on fossils, or from those of Huxley, Cope and Gegenbaur on comparative anatomy. As the result of this change of thought we now see most medical schools prescribing biological courses, and choosing their professors of anatomy largely from the ranks of embryologists.
It is hardly necessary to state that it was Louis Pasteur who laid the foundation for the study of disease-producing organisms; indeed, he may be said to have done more for the human race, more to prevent physical misery, than any other man of the nineteenth century. He had in mind, first of all, the cure, but he realized that to accomplish this the mode of transmission of the disease must be understood. There have followed him a long line of investigators of bacterial diseases, and among them the purely scientific have done quite as much as the purely practical. In Russia there was a celebrated embryologist, Elias Metchnikoff, who worked out the life histories of a variety of animals, and was thereby led to a consideration of the part that the white blood cells play in the development. This brought him to the view that such cells are the guardian policemen of the body, that seek out and destroy the bacteria; and this to the further idea, that health is to be maintained and infection prevented by keeping the white blood cells in proper numbers and activity. Metchnikoff succeeded Pasteur at Paris, and though his theory of phagocytosis is far from all-sufficient, it has nevertheless strongly stimulated the study of bacteriology. His