In order to explain the phenomena of embryonic induction, the isolation of a pure inductive agent is the most fundamental requirement. As shown by the present authors ( ' 6 0 ) , chick embryo extract may be said to be one of the most suitable source materials of the embryonic inductive-agents, because not only can a solely neural-inductive capacity be found in the ribonucleoprotein (RNP) fraction of the extract, but the possible existence of a solely mesodermal-inductive agent can be anticipated in the non-ribonuclee protein (non-RNP) fraction from the same extract (Tiedemann, '59).Actually solely mesodermal inductions consisting of notochord, myotome and mesenchymatous tissue are frequently produced even by using as the inductor material the non-RNP fraction separated with streptomycin from the chick embryo extract, as well as a similar fraction separated with sodium deoxycholate from the microsomal fraction of chick embryos (Kawakami, Iyeiri and Matsumoto, in press), although a majority of the cases develop neural tissues together with the mesodermal ones. On the other hand, if the view that the spinocaudal structures are realized by a co-operative inductive action between the neural-and mesodermalinductive agents (Toivonen and S a x h , '55; Kawakami and Yamana, '58) is accepted, it may be possible to separate the two agents from the non-RNP fraction of the chick embryo, which shows a strong spinocaudal-inductive effect.Tiedemann and Tiedemann ('59) have already succeeded in purifying inductive agents from chick embryo homogenates by various kinds of procedures. The agent which was separated by phenolpyridin fractionation showed an especially prominent mesodermal inductive effect. Moreover, the experiments of these investigators on inductive agents cover a wide scope, and have produced noteworthy results.The present authors, however, have performed the analysis and separation of the inductive agents by paying special attention to the morphogenetic and cytophysiological relations between the mesodermal and neural-inductive agents and to the distribution of these agents among cell components. They have therefore tried to separate out all the agents contained in chick embryo extract which are effective in producing induction (Kawakami, Iyeiri and Sasaki, '60; Kawakami, Iyeiri and Matsumoto, in press).A protein fraction salted out at Y 3 saturation of ammonium sulfate ("Ih-Fraction" in the previous paper) from the nun-RNP fraction, which is separated by precipitating the RNP fraction from the chick embryo extract with streptomycin, gives a very strong spinocaudal-inductive effect; moreover, solely mesodermal inductions occur more frequently than those obtained by the original non-RNP (Kawakami, Iyeiri and Sasaki, '60). This fraction may be of use, therefore, as a source of the specific agent for neural or mesodermal induction. In the present work the isolation of pure inductive agents from the %-Fraction was therefore attempted.
