PRENATAL GROWTH OF THE PIG 



123 



The rapidity of growth, however, is quite different in the vari- 

 ous animals at corresponding periods. Thus, at the end of three 

 weeks of prenatal life, the pig has increased about twenty times 

 as rapidly as has the human embryo, but only about one-twentieth 

 as rapidly as the white rat and one-fiftieth as rapidly as the rabbit. 

 After birth of the rat and rabbit, however, their growth rate is 

 greatly decreased; so that at 112 days the pig (newborn) has far 

 outstripped the rat (275 million to 47 million), and is approaching 

 the rabbit, which has reached a weight of about 333 million times 

 the weight of the ovum. The human embryo still lags behind 

 (30 million) but is approaching the rat. At one year postnatal 

 (sixteen months total) the pig has reached 33 billion times the 

 weight of the ovum. Next in order is the human (seven months 

 old) with an increase of two billion times, while the rabbit and 

 white rat have dropped far behind. Thus the animals with a 

 short gestation period (rabbit, rat) have a more rapid grow^th rate 

 during that period, but are thereafter overtaken and outstripped 

 by those with a longer gestation period (pig, human). 



At the time of birth, the growth rate always undergoes a rapid 

 decrease, the prenatal rate always being far more rapid than the 

 postnatal. There seems to be no definite relation, however, 

 between the length of the gestation period and the ratio of new- 

 born to adult weight. Of the five species considered, the human 

 is relatively nearer the adult weight at birth, the ratio being about 

 1:19. The pig is farthest from the adult, the ratio being about 

 1:120. The rabbit, rat and chick take an intermediate position, 

 their ratio being about 1:60 (table 3). 



THE A.MERICAN JOURNAL OF ANATOMY, VOL. 12, NO. 2 



