706 



THE PHYSIOLOGY OF REPRODUCTION 



males. Both show an early period of rapid decline in which the rate 

 of growth is quickly diminishing, followed by a period of slight 

 decline in which the curve is still falling, but very much more 

 gradually (Figs. 175 and 176). 



Minot has also investigated the rate of growth in the rabbit and 

 in the chicken.' The young rabbit, as is well known, is born in a 

 very immature state of development after a relatively short gestation 

 period. Correlated with this fact, it was found that the male rabbit 

 four days after birth is capable of adding over seventeen per cent, to 

 its weight in a single day. From that time the percentage increment 

 drops very rapidly, so that at an age of twenty-three days the rabbit 

 can only add a little over six per cent. After about the fifty-fifth 

 day the decline in the growth rate, which has hitherto been rapid, 



2 sen " a a xx 



FIG. 176. (From Minot's Problem of Age, Growth, and Death, 

 G. S. Putnam & Sons, and John Murray. ) 



becomes more gradual. In the case of the chicken, Minot's results 

 were in a general way similar, but the rate of growth on the first 

 day it could be measured was a nine per cent, addition to the weight, 

 while the change from the initial rapid decline to the subsequent 

 slow decline was more gradual than in the other two animals. 1 



The mean weight of the foal at birth is said to be 112 Ibs. 

 During the first three months the average daily increase is 2'2 Ibs. ; 

 from three up to six months it is 1*3 Ibs. ; and from six months 

 up to three years 0'7 lb. It is said that probably many horses 

 continue to grow until they are six years old. 2 



The calf at birth weighs about 77 Ibs., and the average daily 



1 For an exhaustive account of the growth phenomena in the rat, in which 

 the growth curve is in a general way similar to that of the rabbit, see Donaldson, 

 The Rat (Wister Institute Memoirs, No. 6), Philadelphia, 1915. 



2 Smith (F.), Veterinary Physiology, 3rd Edition, London, 1907. 



