deal with the invertebrates at all. This bald statement may give you some impression of the present vast extent of the science.
What I wish to attempt on this occasion is to select out of this huge accumulation of discovery some illustrations of the way in which embryology has made contributions of practical value to medical science and medical practitioners. I have on another occasion spoken of the relations of science and the scientific spirit to medical education and practise, and on yet another occasion have discussed embryology as a basis of pathology, so that it seems unnecessary to deal again and before you with these more general aspects of the situation, but I shall ask your attention rather to certain more detailed and specific examples.
Every science has its larger aims and purposes. Those of embryology may be classed not unnaturally under five heads. First, I shall group together those researches that refer to the general topic of generation, the production of the new being, the conditions under which it first develops, including, of course, for man especially, the relations of the fruit to the womb. Under this head are comprised phenomena of impregnation, problems of heredity, the origin of sex, the conditions of gestation and pregnancy, and the physiological causes of birth.
Secondly, under the head of cytomorphosis we can put all the work which has been accomplished in tracing out the development of cells. The conception of cytomorphosis is one which has only recently become clear to us, but it is one of the most fundamental notions of biological science, and one which every student of morphology, pathology or physiology must clearly grasp and keep constantly in mind. Cytomorphosis has been defined as the comprehensive designation for all the structural modifications which cells or successive generations of cells may undergo from the earliest undifferentiated stage to their final destruction. It starts with the history of the undifferentiated cell, considers all phases of differentiation, and in those cases where the process goes to its end, it follows out the final steps of the degeneration and destruction of the cell. The law of cytomorphosis is indeed the chief foundation of all anatomical and pathological science. The possibilities of modification within a cell are determined by the stage of cytomorphosis which is reached, and as it goes forward from stage to stage the possibilities of further change become more and more limited in accordance with the recently established law of genetic restriction. I have expounded my views on the importance of the laws of cytomorphosis for pathology in the Middleton-Goldsmith lecture for 1901 and need not now dwell longer upon the subject.
Third, I should class the studies which refer to the germ layers, those laminæ of cells which, as it were, occupy an intermediate place between the single cell and the organ. They correspond to the first orderly arrangement of cells which we have in the organism, and from
