Studies in fetal metabolism.

The origin of the constituents of the mammalian fetus has been the subject of controversy, the literature of which has been exhaust,ively reviewed by Needham (1, 2) and Huggett (3). Two processes come into consideration: transplacental absorption from the maternal circulation and synthesis in t,he tissues of the fetus proper. In regard to the transplacental passage of fat,ty acids, the most impressive evidence has been the finding of elaidic acid in the fetal fat after the feeding of this abnormal fatty acid to pregnant animals (4, 5). Unequivocal proof for the synthesis of fatty acids (6) and cholesterol (7) in the chick embryo has been adduced, but similar evidence in the mammalian fetus is lacking. There appears to be no reasonable doubt that glucose crosses the placenta, and the relative abundance of fetal glycogen, compared with concentrations in adult tissues, has repeatedly been observed. Whether t,he latter results from excessive glycogen deposition or lack of glycogen mobilization has not been determined. The view that. epinephrine crosses the placenta has been favored by some workers and contested by ot.hers. When epinephrine was administered to pregnant animals, no fall in fetal glycogen content was detected (8). Similarly the transplacental passage of insulin has been the subject of controversy. The most frequent argument has centered about the increase in glucose tolerance incident to pregnancy in the diabetic (9, lo), but this may be ascribed t,o the large consumption of maternal glucose by the growing fetus and need not indicate passage of fetal insulin into t(he maternal circulation (11). The isot,opic tracer technique seems to be well adapted to the study of some of these problems. The experiments here reported have been designed to investigate (a) the transplacental migration of certain body constituents, (b) the fetal synthesis of certain constituents, and (c) the con-