this tripartite arrangement the organs are fashioned. Of course, the fundamental morphological fact in regard even to the higher animals is the cell, but next to that we may place the existence of the germ layers, the complicated interrelations of which dominate the entire history of every individual alike in health and in disease. The comprehension of the morphological importance of the germ layers and their relation to the production of tissues and organs and abnormal growths of all kinds is absolutely indispensable to every medical man who wishes to have an intelligent mastery of his subject.
Fourth, we may place the strictly anatomical aspects of embryology, which give the morphological interpretations of organs and the explanation, as you know, of many anomalies of adult structure. The anatomy of the adult offers to us many riddles, for numerous are the arrangements and characteristics of the body which we can not understand or explain from the study of the adult alone. The language of adult structure we often can not read unless we have first studied the Rosetta Stone of embryology which affords us the key of translation. As a teacher in a medical school, I have again and again been profoundly impressed with the value of embryology to the student of anatomy. Things which are obscure are illuminated by a knowledge of the developmental changes. In an embryo we encounter simplified conditions; secondary modifications coming in later in the course of development not only add to the complication of parts, but often also produce so great changes as to mask the fundamental and original relations. What student of adult anatomy alone could possibly discover that the thymus gland is a modification of the lining epithelium of a gill pouch which exists as a pouch in the embryo and is homologous with the gill pouch of a fish? Or what pure anatomist could ever have discovered that the spermiduct is the modified duct of a kidney present in the embryo, but which in the adult has as such totally vanished? If we pass from mere human anatomy to the larger and more scientific subject of comparative anatomy, we feel again the value of embryology, which establishes the real homologies of structure, proving exact homologies from the study of the early stages of parts, which in different types become so unlike that their fundamental identity of origin is completely hidden. For example, without embryology we never should have known that the little bone of the ear which we call the malleus is homologous with the upper part of the lower jaw of a cartilaginous fish. Indeed, the stories which embryology has to tell are the most romantic known to us, and the wildest imaginative creations of Scott or Dumas are less startling than the innumerable and almost incredible shiftings of role and changes of character which embryology has to entertain us with in her histories. I have been tempted to exclaim sometimes while pursuing my science that in embryology only the unexpected happens.
