222 AGE, GROWTH, AND DEATH 



index is n, in the general connective tissue of the 

 body 10; for the cells of the liver n; in the outside 

 layer of the skin 10 ; in the excretory organ 6 ; in the 

 tissue which forms the centre of the limb also 6. 

 There has, then, been a rapid decline in the rate of cell 

 multiplication just in this period when differentiation 

 is going on. This is, so far as I know, an entirely 

 new line of research. The counting of a thousand 

 cells is not to be done very rapidly ; it must be under- 

 taken with patience, care, and requires time. It has 

 not, I regret to say, been possible for me yet to ex- 

 tend the number of these counts beyond those I have 

 given you, but it is safe to say that in the yet 

 more differentiated state, the number of cells in divi- 

 sion is constantly lessened, and it is only a question 

 of counting to determine the mitotic index accurately. 

 That there is a further diminution beyond that which 

 the mitotic indices I have demonstrated to you repre- 

 sent is perfectly certain. I only regret that I am not 

 able to give you exact numerical values. 



I wish very much that my time permitted me to 

 branch off into certain topics intimately associated 

 with the general theme we have been considering 

 together on these successive evenings, but we can 

 only allude to a few of these. The first collateral 

 subject on which I wish to speak to you briefly is 

 that which we call the law of genetic restriction,' 1 which 



1 C. S. Minot, Laboratory Text-book of Embryology (1903), p. 30. This law 

 of genetic restriction had been foreshadowed by M. Nussbaum (Arch.f. microsk. 

 Anal., xxvi., pp. 522, 524). He expresses well and ingeniously the change of 

 progressive differentiation in Metazoan cells. His idea is that each cell is a 



