400 ON INCREASE IN SIZE [pt. iii 



the work of those who have infused more theory into their treatment 

 of the experimental facts. 



Before 1890 there was no regularity in the way in which experi- 

 mentalists examined their data on growth. But about that time 

 Minot began a long series of investigations on the growth of 

 animals, mainly the guinea-pig and the rat, in which he introduced 

 a new method, namely, the evaluation of the growth-rate by taking 

 it as the increment in per cent, of the weight of the animal at the 

 beginning of the period in question. Some workers, e.g. Preyer, had 

 already adopted this plan. The percentage growth-rate has always 

 been found to decline enormously as development proceeds, an 

 observation which led Minot to say that the embryo gets oldest 

 most quickly when it is youngest. This apparently paradoxical 

 statement drew a good deal of attention to his work at the time, and 

 his book. The Problem of Age, Growth, and Death, included many such 

 graphs showing how rate of senescence was greatest in the earliest 

 periods. One of these is reproduced here as Fig. 43. 



Another contribution of Minot's was the conception of the "mitotic 

 index", or the number of mitosing cells per 1000 cells in a tissue. 

 He did not himself find time to do more than a few of these laborious 

 counts, but he gave the following figures, which showed that the 

 mitotic index declined with age: 



These data lent weight to his principal conclusion, which was that 

 the younger the embryo the more rapidly it aged. Practically 

 nothing more was done along these lines until Olivo & Slavich in 

 1929 reported a large series of figures for the mitotic index of the 

 developing heart in the chick. 



