Twenty-four hours after the intraperitoneal injection of sodium sulfate-S35 into pregnant rats, sulfur-35 was found in the embryos. The amount of the sulfur-35 retained by the embryos was directly related to their degree of development in utero.

A large fraction of the sulfur-35 found in the embryos was insoluble in 5 per cent trichloroacetic acid. At the 9th to 10th day of development, about 40 per cent of the sulfur-35 was present in this fraction. In 20-day-old embryos this fraction accounted for nearly 90 per cent of the total.

Radioautographs of sections of embryos fixed in a solution of formaldehyde revealed that the sulfur-35 was most highly concentrated in the cartilaginous portion of the skeleton. All other tissues gave much weaker autographic reactions, comparable with the over-all reaction obtained when sections from embryos fixed in a solution of formaldehyde saturated with barium hydroxide were used.

By analysis for the sulfur-35 content of individual tissues the concentration of the sulfur-35 in humeri from 20-day-old embryos was found to be about 30 times that in the maternal sternum. The concentration of the isotope in the skeletal muscle, brain, heart, and skin of the same embryos was also higher than in the corresponding maternal tissues. On the other hand, the concentration of the sulfur-35 in the maternal gastrointestinal tract plus contents was higher than in the gastrointestinal tract and contents of the embryos.

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