EMBRYOLOGY AND MEDICAL PROGRESS 15 



from our human point of view at the wrong time and in the wrong 

 place, but which are not of themselves wrong or diseased, though they 

 become so in man's nomenclature by their mode of occurrence. This 

 broader conception of degeneration affords a new foundation for 

 further investigation, and by the hearty cooperation of the embryologist 

 and the pathologist we may expect new enlightenment. 



The fourth head under which we classed the work of embryologist 

 corresponded to the field of anatomical research. We all know that 

 the embryological explanation of the anatomical disposition in the 

 adult is a real and clarifying explanation. Certainly no teacher of 

 anatomy to-day, competent to his work, will undertake to teach the 

 structure of the brain, of the urogenital system or of the heart, except 

 on an embryological basis. But there are a great many other an- 

 atomical conceptions which may best be made clear if we start with an 

 examination of the conditions in the embryo. My experience as a 

 teacher has afforded me many examples of this. Let me mention a 

 few. The arrangement of the great cephalic nerves is a subject of 

 peculiar difficulty to the student, but by the examination of a few 

 properly chosen sections through the head of mammalian embryos all 

 the essential topographical relations can be made easily understandable, 

 and these essential relations are never obliterated by any further de- 

 velopment. The disposition of the peritoneum is one of the greatest 

 bugbears to the first-year medical student. But let him study the 

 peritoneum in its relation to the viscera in the young embryo and he 

 easily overcomes his difficulties and gets a clear and correct conception 

 of the topographical relations of the peritoneal membrane and is able 

 thereafter to comprehend the secondary modifications by which the 

 adult topography is so much complicated. So too in regard to the 

 thorax, a few sections from embryos give definite and exact conceptions 

 of the fundamental relations of the heart and lungs, the mediastinum, 

 and of the pleural and pericardial membranes. A good student may 

 obtain from such a section a visual image which he will carry with him 

 throughout life and which will always serve to make clear in his mind 

 all these anatomical relations. One more similar instance may suf- 

 fice. Students are always perplexed by the nature and mutual con- 

 nections of the three membranes of the spinal cord and brain. Here 

 also experience convinces us that sections of embryos reveal the facts 

 so perfectly that they are readily comprehended and not easily for- 

 gotten. But I think I need not argue further to convince you that 

 embryology as an aid to anatomical study is of incalculable value, and 

 ought, if we are to do our anatomical teaching conscientiously, to be 

 included in every medical curriculum. 



Not infrequently the study of embryos establishes entirely new 

 anatomical conceptions. An instance of this is offered by the study 

 of blood vessels. We have learned in recent years that in addition to 



