NINETEEN FIGURES
SPINO-CAUDAL INDUCTION
293:is responsible for the cephalo-caudality of the obtained induction in such a way that the stronger its action, the more caudal the level which will be induced. If we adopt the termisology of regionality, which is based on a suggestion of I?. E. Lehmann ('45, p. 234) and used in the current literature by Toivonen, Kuusi and others, the idea can. be explained in the following way. The structures like eye, fore-brain, and nose, which are normally derived from the anterior part of the primary neural plate and its vicinity and are collectively called "archencephalic," will be induced in the presumptive ectoderlm, if Mdv alone acts on the ectoderm. The,,structures like somites and spinal cord, which are derived from the posterior part of the primary neural plate and are collectively called "spinocaudal," will be induo th of the factors, Mdv and Mcc are present and the of Mcc is maintained above a definite level. The st like mid-brain, hind-brain, and ear vesicle, which are derived normally from the intermediate part of the primary neural plate and its vicinity and are collectively called "deuterencephalic, " will be induced, if Mdv and Mcc are both present, but the activity of Mcc is less than the level above mentioned.These mediators cannot be called regionally specific inducing factors, because the inductive effect is not decided according to a simple one-to-one correspondence between the inducing factors and the regional types of induction. However Mcc