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Mesodermalizing Factors in Fetal Calf Serum

The inducing activities of fetal calf serum (FCS) and fetal calf serum heated for a short time (HFCS) were tested with early gastrula presumptive ectoderm of the newt, Cynops pyrrhogaster as a reactor. Inducing activity was measured by sitting‐drop culture of pieces of ectoderm before and after treatment with Ca, Mg‐free solution (CMF) in culture medium contained FCS or HFCS (10–50%) as inductor. Results showed that FCS did not induce mesodermalization of ectoderm, before or after treatment with CMF, and that HFCS induced mesodermalization only of ectoderm treated with CMF. The treated ectoderm cells differentiated into mesoderm cells such as muscle and notochord. The induction increased with increase in the duration of CMF treatment and with the concentration of HFCS in the medium. This indicates that FCS changes into mesodermalizing material when heated for a short time and that mesodermal induction by HFCS depends on some effect of CMF on presumptive ectoderm. Since CMF was found to be a neuralizing factor, activation of presumptive ectoderm with a neuralizing factor is probably a prerequisite for mesodermal induction with HFCS.