84 AGE, GROWTH, AND DEATH 



Old age, after what I have said, I think you will 

 all recognise as merely the advanced and final stage 

 of cytomorphosis. Old age differs but little in its 

 cytomorphosis from maturity ; maturity differs much 

 from infancy ; infancy differs very much indeed from 

 the embryo ; but the embryo differs enormously from 

 the germ in its cytomorphic constitution. We know 

 that in the early time comes the great change, and 

 this fact we shall apply for purposes of interpretation 

 later on. Cytomorphosis is then a fundamental no- 

 tion. It gives us in a general law, a comprehensive 

 statement of all the changes which occur in the body. 

 None, in fact, are produced at any period in any 

 of us except in accordance with this general cyto- 

 morphic law. There is, first, the undifferentiated 

 stage, then the progressive differentiation ; next there 

 follows the degenerative change ending in death, and 

 last of all, the removal of the dead cells. Such we 

 may conveniently designate as the four essential 

 stages of cytomorphosis. This cytomorphosis is at 

 first very rapid ; afterwards it becomes slower. That 

 is a significant thing. The young change fast ; the 

 old change slowly. We shall be able, when we get a 

 little farther along in our study, to see that in differ- 

 entiation lies the explanation of a great many of the 

 known phenomena of biology, lies the explanation of 

 our conception of cell structures ; and in it also lies 

 not only the explanation of the death of cells, but 

 also, as it seems to me and this is one of the points 

 that I shall want particularly to bring forward before 



