MITOTIC ACTIVITY AND CELL PROLIFERATION IN PRIMARY INDUCTION OF NEWT EMBRYO

Mitotic activity and cell proliferation of newt (Triturus pyrrhogaster) embryo were examined with special reference to primary induction. Mitotic activity of gastrula ectoderm gradually decreases during gastrulation. The ectoderm, which is isolated from mid-gastrula (stage 12b) and cultured in vitro, also shows gradual decrease in mitotic activity during cultivation and the mitotic activity steeply decreases after 48 hr. The ectoderm cultured with heterologous inductor (GPL-extract) shows a temporal suppression in mitotic activity. The ectoderm of the whole gastrula also shows a regional suppression where it is in contact with the chorda-mesoderm. The number of the ectodermal cells increases about 2 times after 24 hr culture and to more than 3 times after 48 hr culture. Accordingly it is certain that the majority of the ectodermal cells divides at least one time in the course of 48 hr. Histological examination of the ectoderm cultured together with the inductor reveals that differentiation of undifferentiated ectoderm to neural tissues is accomplished at least within 48 hr after cultivation with the inductor. The present examination shows the possibility that the mitotic activity of the ectoderm may be temporarily suppressed by the inductor and that it then decreases along with neural cell differentiation after recovery of the activity. The results also suggest that the determination of undifferentiated ectoderm to neural tissues occurs before the second cell division after the contact with the inductor and the events occurring during the first cell cycle after activating by the inducing stimulus are critical for the primary induction.