io POPULAR SCIENCE MONTHLY 



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 em- 

 bryology 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, pa- 

 thology 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 degenera- 

 tion 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 lec- 

 ture for 1901 and need not now dwell longer upon the subject. 



Third, I should class the studies which refer to the germ layers, 

 those laminse 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 



