REGENERATION AND DEATH 189 



the cerebellum, that we find cells exist such as I have 

 just described to you, but it is in other parts of the 

 body also and at other periods of life that we find the 

 like phenomena ; and in part I have already referred to 

 these. You remember I told you in a previous lecture 

 that there is always in the body, even at the extreme 

 of life, a store of cells of the young type, which is 

 garnered in the marrow of the bones. The cells in 

 question can multiply, and their descendants in part 

 undergo a change in consequence of which they are 

 converted into blood corpuscles. The undifferentiated 

 or young cells are preserved in the marrow precisely 

 for the purpose of making up the necessary number of 

 blood corpuscles to replace those which are lost either 

 by accident or in consequence of normal physiological 

 processes. 



We can speak in more general terms. In the very 

 early stages of the embryo the growth is diffuse, or 

 as it is sometimes termed technically interstitial. Of 

 course growth depends upon cell multiplication, and 

 when we say growth is diffuse, we mean that cell di- 

 vision takes place throughout the organ or tissue. For 

 example, in the three germ layers, in a stage preced- 

 ing the differentiation of organs, we find the mitotic 

 figures (which prove active cell division) to be scat- 

 tered about. They may occur in any part of each layer. 

 As development progresses the growth becomes in 

 many parts focal, that is to say, there are established 

 centres of growth, at each of which the multiplication 

 of cells proceeds, while in the immediately surround- 



