II. 2 GROWTH 65 



diminution is gradual. A point of importance is that in both 

 years there is a slight temporary rise in the growth-rate about the 

 time of puberty (see the curve, Fig. 37). This has been noticed by 

 all observers, but the actual time of its occurrence differs in differ- 

 ent cases ; the rise is invariably earlier in females than in males. 

 A comparison of the growth of the three mammals considered 

 is interesting. 



A Guinea-pig reaches 775 grammes in 432 days. 

 A Rabbit 2,500 395 



A Man 63,000 9,428 



or the average daily increment is for a 



Guinea-pig 1-82 grammes. 

 Rabbit 6-30 

 Man 6-69 



Hence 'men are larger than rabbits because they grow longer, 

 but rabbits are larger than guinea-pigs because they grow 

 faster'. Minot, however, distinguishes between the 'rapidity' 

 of growth, the average actual increment, and the ' rate ' of 

 growth, the percentage increment. The average percentage 

 increments for these mammals are 



Guinea-pig 0-47 

 Rabbit 0-50 

 Man 0-02 



The rate is, therefore, much slower in man than in the other 

 two. These percentages Minot calls the coefficients of growth. 

 Together with the duration of growth they determine the ultimate 

 size of the organisms. 



The progressive loss of growth-power Minot speaks of as 

 1 senescence ', and compares to the loss of the power of cell- 

 division in the ' senile decay ' of Protozoa. The same author 

 has also brought forward evidence to show that during differ- 

 entiation there is an increase in the amount of cytoplasm in the 

 cell, a decrease in the size of the nucleus, and a decrease in the 

 'mitotic index', that is in the proportion, in any tissue, of 

 dividing cells. During segmentation, of course, the reverse of 

 this is taking place, since cell-division is rapid and the protoplasm 

 per cell is being constantly diminished until a fixed ratio between 



