BRAIN INDUCTION IN VARIOUSLY AGED PRESUMPTIVE ECTODERMS BY HEAD ORGANIZER IN CYNOPS PYRRHOGASTER

Brain formation in variously aged presumptive ectoderms of Cynops pyrrhogaster under the influence of the head organizer was examined by the sandwich method. The head organizer was obtained from the middle portion of the archenteron roof at the slit-blastopore stage. The presumptive ectoderm was taken from 0- to 36-hr exogastrulae. Exogastrulae were prepared from the earliest gastrulae just before invagination (0-hr embryos). The presumptive neural plate overlying the archenteron roof used as organizer was cultivated in an envelope of belly ectoderm from an early neurula. The following results were obtained: 1) Brain induction was almost entirely restricted to explants covered with 6-hr ectoderm and its frequency was low. 2) The presumptive neural plate above the head organizer was almost completely determined as neural tissues. 3) The head organizer showed a tendency to differentiate into more endodermal and less mesodermal tissues than those expected from its prospective fate. Brain induction in normal development and the relationship between neural tissue formation in variously aged presumptive ectoderms and the time necessary for neural induction are discussed.