The increase in length, wet weight, dry weight, DNA content and chondroitin sulfate content of the tibiotarsus of the embryonic chick has been compared in vivo and in uitro on a chemically defined medium. Both the rate and magnitude of increase were lower in tibiotarsi in organ culture than those in the whole animal.
Comparison of ratios of various growth responses gave some indication of the mode of growth in the two situations. The dry weight/wet weight ratio of tibiotarsi in viuo remained virtually unchanged during the growth period in this study, while the ratio of tibiotarsi in uitro declined continually during the cultivation period. The hydration of skeletal elements growing in culture is probably due to degenerative phenomena. The ratio of dry weight to DNA showed that the cells of tibiotarsi in culture did not synthesize dry matter as rapidly as those in the whole chick. On the other hand, the ratio o f chondroitin sulfate to DNA indicated that the cells of tibiotarsi in culture were producing chondroitin sulfate at a rate similar to that in vim. Furthermore, the proportion of chondroitin sulfate in vitro tended to be greater than in the chick embryo. These findings suggest that the matrix of skeletal elements growing in culture, although chondroitin sulfate rich, may be deficient in one or more matrix components.