THE FOUR LAWS OF AGE 221 



given number which have nuclei in process of divis- 

 ion, and such a number I propose to call " the mitotic 

 index." 1 I wish now only to call to your attention this 

 picture because it enables me to illustrate before you 

 the method of measuring the mitotic index. 



In the rabbit embryo at seven and one half days, I 

 have found by actual count that there are in the outer 

 layer of cells, known technically as the ectoderm, 18 

 of these divisions per thousand ; in the middle layer, 

 technically the mesoderm, 17, and in the inner layer, 

 the entoderm, 18. At ten days we find the number 

 already reduced, and the figures are, respectively, 14, 

 13, and 15, and for the cells of the blood only 10. 

 There has already been a great reduction. In the 

 next phase of development (rabbit embryo of thirteen 

 days), we find, however, that the parts are growing 

 irregularly, some faster, some slower. We note that 

 wherever a trace of differentiation has occurred, the 

 rate of growth is diminished ; where that differentia- 

 tion does not show itself, the rate of growth may even 

 increase in order to acquire a certain special develop- 

 ment of a particular part. So that instead of uni- 

 formity of values for the mitotic index, we get a great 

 variety. But, nevertheless, the general decline can be 

 demonstrated by the figures. In the spinal cord the 



1 Cells are known to divide without mitotic figures appearing ; the process is 

 then termed amitosis. Amitosis occurs in certain degenerating cells of mam- 

 mals and is said to occur in sundry invertebrates as a normal process. So far as 

 I am aware there is, however, at least as yet, no evidence that amitosis occurs 

 in the embryos of the higher vertebrates. If it did occur it might diminish the 

 validity of the mitotic index. 



