114 EFFECTS OF INANITION IN THE PREGNANT ALBINO RAT. 



weight of the thymus in the newborn as 0.0079 gram or 0.15 per cent of the body- 

 weight — 5.03 grams. The Wistar norm (Donaldson, 1915) is 0.00S0 gram, body- 

 weight, 4.9 grams. 



In my prenatal controls the thymus forms 0.11, 0.13, 0.13, 0.13, and 0.12 per 

 cent of the body-weight in Groups I to V, respectively (computed from table 5) . 

 Thus the relative weight of the thymus in the prenatal controls is slightly lower than 

 in the normal newborn rat. 



In my test rats the thymus forms 0.08, 0.09, 0.11, 0.11, and 0.10 per cent of the 

 body weight in Groups I to V, respectively (computed from table 5). Thus the 

 thymus forms a much smaller part of the body-weight in the test rats than in the 

 prenatal controls; that is, its growth has been retarded in the test rats. This is 

 more clearly brought out by a comparison of the absolute weights. In Group I the 

 absolute weight ot the thymus, in the test rats, is 29 per cent below that of the pre- 

 natal controls; in Group II, 33 per cent below; in Group III, 12 per cent below; in 

 Group IV, 16 per cent below; and in Group V, 15 per cent below, averaging 21 per 

 cent below for all groups. Thus it is seen that the growth of the thymus is con- 

 siderably retarded in the rat fetus during inanition of the mother. 



These results also agree with the loss of weight (hunger involution) of the 

 thymus in cases of postnatal inanition. Jonson (1909), in young rabbits, kept a 

 constant body-weight by underfeeding for 4 weeks, found the weight of the thymus 

 to be reduced to one-thirtieth of that in the controls, the greatest loss of weight 

 being in the cortex. He also found that the reduction in weight of the thymus was 

 proportionate to the loss of the body-fat. This reduction in weight of the thymus 

 in rabbits is much more marked than that obtained by Jackson and Stewart upon 

 rats. Jackson (1915a) found a loss of 90 per cent in the thymus in young rats held 

 at maintenance from the age of 3 to 10 weeks and also in young rats underfed 10 

 weeks to 8 months. Stewart (1918 and 1918a) found losses of 80, 30, and 49 per 

 cent, respectively, in the weights of the thymus in young rats underfed from birth 

 to 10 weeks, from birth to 3 weeks, and held at birth-weight for 16 days. Jackson 

 (1915) found no marked change in the weight of the thymus in cases of acute and 

 chronic inanition in adult rats, at which age involution of the thymus had already 

 occurred. The thymus normally reaches its maximum absolute weight in the rat at 

 85 days (Hatai, 1914) and at 1 year it has undergone a complete age involution 

 (Jackson, 1913). 



Heart. — In my normal newborns, the average weight of the heart is 0.027 gram 

 or 0.55 per cent of the body-weight, 4.92 grams. Jackson (1913) gives its weight in 

 the newborn as 0.030 gram, or 0.65 per cent of the body-weight, 5.08 grams. Stew- 

 art (1918a) gives a weight of 0.031 gram, or 0.61 per cent of the body-weight, 5.03 

 grams. 



In my prenatal controls the weight of the heart forms 0.49, 0.52, 0.49, 0.50, and 

 0.44 per cent of the body-weight in Groups I to V, respectively (computed from 

 table 5). Thus the relative weight of the heart in the prenatal controls is con- 

 siderably less than in the normal newborn rat. 



