PRENATAL GROWTH OF THE PIG 129 



In general, the relative size and course of growth of the thymus 

 appears somewhat similar in the pig and in the human. 



The lungs. {Table 1: fig. 3). The lungs are considered to- 

 gether because, while there is a difference in size, their ratio to 

 each other is fairly constant. The curve for both lungs together 

 is given in fig. 3. Forming 0.6 per cent of the total weight at 

 18 mm., the lungs increase rapidly in relative size until, at 86 

 nam., they form 3.89 per cent of the total body weight. From 

 here the decrease in relative size is somewhat irregular to the late 

 fetus, where they form about 2 per cent of the total weight. In 

 the adult they form about 0.69 per cent. In the smallest litters 

 examined, there is apparently no appreciable difference in size 

 between the two Itings. The difference between the two lungs 

 in the later stages, while holding fairly constantly to the ratio 

 (right to left) of 7 :5, is liable to considerable individual variation, 

 as is the relative size of each lung. 



Approximately the same course of growth occurs in the human. 

 There is an initial rise, the maximum relative size occurring in 

 the fourth fetal month, when the lungs average 3 . 29 per cent of 

 the total weight. In the still-born they average 1.7 per cent; 

 in the live-born, 2.18 per cent. In the adult they form 1 . 5 per 

 cent of the total ( Vierordt) . In relative size therefore, the human 

 lungs are approximately equal to those of the pig during prenatal 

 life, but are about twice as large in the adult. In both, the maxi- 

 mum occurs early in prenatal life. 



The right lung in the human is also larger than the left, averag- 

 ing about 20 per cent larger. In the adult, the right lung is 

 usually stated to be only 10 per cent larger than the left. In the 

 pig, however, the right lung averages about 40 per cent greater 

 than the left, both in the fetus and the adult. 



The heart. {Table 1: fig. 2). The heart has its greatest ob- 

 served relative size in the earliest stage examined (15 mm.), 

 forming 4.64 per cent of the total weight. An examination of 

 earlier stages would possibly show a still larger maximum. It 

 decreases in relative size rapidly at first, then more slowly, until 

 at 125 mm. it forms only 0.66 per cent of the total body weight. 

 In the later fetuses there is a gradual rise, the heart forming 1 .03 



