EMBRYOLOGY AND MEDICAL PROGRESS n 



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 com- 

 prehension 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 ex- 

 planation, 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 under- 

 stand 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 

 Eosetta Stone of embryology which affords us the key of translation. 

 As a teacher in a medical school, I have again and again been pro- 

 foundly 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 dis- 

 cover 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 homol- 

 ogous 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, prov- 

 ing 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 crea- 

 tions 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 em- 

 bryology only the unexpected happens. 



