PRENATAL GROWTH OF THE PIG 125 



as the brain and cord: at birth they are more than three times 

 as large; while in the adult, they are about ninety times as large. 



3. Relative growth of the various organs 



The relative growth of the organs together having been shown, 

 it is now necessary to take up the growth of each individual organ 

 separately, as their growth is by no means uniform. 



The head. Table 1 and the head curve in fig. 1 show that 

 the head was found relatively largest in the second stage examined 

 (18 mm.), forming on the average, 29.69 per cent of the total. 

 Throughout the earlier fetal stages it forms from 20 per cent to 

 28 per cent of the total, and in the later averages about 22 per 

 cent to 23 per cent. Its minimum observed relative size is 19 . 65 

 per cent in the 108 mm. litter. In the nearly full term fetus it 

 averages 22.3 per cent of the total. In the adult it forms 6.26 

 per cent of the total body weight. The decrease in the relative 

 size of the brain is much larger than that of the entire head, so 

 that the facial structures form an increasingly larger portion of 

 the head in the later fetuses and the adult. 



In the human, the head reaches a maximum relative size of 

 45 per cent during the second month. It then decreases gradually 

 in relative size, forming about 26 per cent of the total at birth. 

 The adult human head forms 6 to 9 per cent of the total body 

 weight. 



The head of the pig at no stage observed reaches as large a 

 relative size as does the human at a corresponding stage, having, 

 for the most part, a relative size about two-thirds that of the 

 human. In the adult, the relative size is more nearly equal in 

 the two species. 



In all vertebrates, from the fishes upward, the embryonic head 

 is relatively large, especially in the early stages. The extent to 

 which this is true varies in the different forms, the head being in 

 general best developed in the amniota. It is perhaps largest in 

 bird embryos, where it may form more than half of the entire 

 body (chick). There is also much variation in different species 

 of mammals, as shown by the difference between pig and human. 



