The localization of sulfur-35 administered intraperitoneally as sodium sulfate to 7-day-old rats was determined by radioautography of sections of humeri and tibia-knee-femur combinations removed 24, 48, 96, 216, and 290 hours after injection of this isotope.

Radioautography of sections of bone and cartilage that had been fixed in formalin indicated that the tracer isotope was present throughout the entire epiphysis. Its concentration therein was highest initially at the epiphyseal-diaphyseal junction where the more mature cartilage cells were present.

By the 96th hour post injection the sulfur-35 had become more uniformly distributed in the epiphyses and an even distribution of it throughout the epiphyseal cartilage was almost attained by the 216th hour post injection.

As centers of secondary ossification arose in the epiphyseal cartilage, the sulfur-35 appeared to diminish in concentration and disappear from these loci. However, radioautographs of cartilage fixed in formalin saturated with barium hydroxide, instead of in formalin only, disclosed the fact that the tracer isotope was still present in these loci.

When bone and bone marrow were fixed in formalin the autographs indicated the presence of sulfur-35 primarily in the periosteum. Only a negligible amount appeared to be present in the bone shaft and marrow. However, when these tissues were fixed in formalin saturated with barium hydroxide it was possible to demonstrate the presence of the tracer isotope in both bone and bone marrow.

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