PRENATAL GROWTH OF THE PIG 131 



In the human, the intestinal tract is relatively small in the early 

 stages, increasing rapidly to full term, when the empty tract forms 

 1.03 per cent of the total weight. The contents here are about 

 twice as great as the empty intestinal tract. Vierordt estimates 

 2 . 06 per cent of the total body weight for the empty adult tract. 



The empt}' intestinal tract of the pig is therefore relatively 

 much larger than that of the human, both in the later fetal stages 

 and in the adult, while the contents are apparentlj- relatively 

 smaller. 



The liver. {Table 1: fig. 1). In the earliest stage examined 

 (15 mm.), the liver forms 12 .25 per cent of the total body weight. 

 In the next succeeding stages it forms the most prominent organ. 

 It has an early maximum, reaching 15 . 88 per cent of the net ho&y 

 weight in the 25 mm. stage and remaining at about this relative 

 size until the 58 mm. stage. From here it decreases sharply, 

 forming 5.01 per cent at 109 mm. In the late fetus it forms 3. 

 per cent of the total (net) body weight. In the adult it forms 

 only about 1.38 per cent of the total body weight. The maxi- 

 mum relative weight may be somewhat more than 15 . 88 per cent, 

 and come just preceding or following the 25 mm. stage. 



In the human fetus, the liver never reaches more than 7 . 5 to 

 10 per cent of the total body, and this maximum size occurs during 

 the second and third months. During the fourth month, the liver 

 drops to an average of about 5 per cent, which it maintains 

 throughout fetal life, averaging 5.23 per cent in live-born infant. 

 Vierordt gives the average for the adult human at 2 . 75 per cent. 

 Therefore, in the earlier fetal stages, the liver has a much greater 

 relative size in the pig than in the human, but in the later fetal 

 stages and in the adult, it is relatively much smaller. 



As is the case in the pig and human, the liver is much smaller 

 in the adult than in the embryo or new born in the chick, dog, 

 stickleback and shrew (Welcker). But it is relatively larger in 

 the adult for the salamander (Welcker) and dog-fish (Kellicott). 



Spleen. {Table 1: fig. 4)- The spleen is liable to very great 

 individual variations in relative size. First observed in the 42 mm. 

 litter, it there has a relative size of 0.016 per cent of the total 

 weight. It gradually increases in relative size, until, in the 26 cm. 



