i 5 8 AGE, GROWTH, AND DEATH 



This is an accurate measure and gives us a good idea 

 of the general law of protoplasmic increase. It is 

 the only instance, I yet know of, in which we have an 

 accurate measure and can give quantitative values, 

 though we do know that there is a more or less similar 

 increase occurring in perhaps every tissue of the body. 



While the increase of the protoplasm is going on, 

 we find that there is an advance in the structure, in 

 the differentiation. Now you may recall what I have 

 mentioned, earlier in this lecture, the further funda- 

 mental fact that the loss in the rate of growth is 

 greatest in the young, least in the old, and that as 

 we go back from old age towards youth, and then 

 into the embryonic period, we find an ever-increasing 

 power of growth, but that it is during the embryonic 

 period that the loss of the power of growth is greatest. 

 It is to the embryonic period, therefore, that I have 

 turned in order to ascertain whether the rate of 

 differentiation shows a similar relation in the de- 

 velopment of the organism. 



We have a large series of microscopic preparations 

 of rabbit embryos in the embryological laboratory 

 of the Harvard Medical School. Utilising these, I 

 found that at seven or eight days of development 

 there is scarcely a trace of differentiation. The cells 

 are in the condition of those which I showed to you 

 earlier in the lecture upon the screen (Fig 47). At 

 sixteen and a half days, a stage of development of 

 which I have some good preparations, I found that 

 a great deal had been accomplished. At seven days 



