Rat embryos were explanted at 12.0 day and cultured for 24 hours in rotating bottles. At the time of explantation they were exposed to culture medium by the opening yolk sac method. Differentiation of the embryos was monitored by counting somites, and protein determinations were made to measure growth. In addition, serial sections were observed with the light microscope to elucidate the inner ear development in-vitro. During the culture period the mean somite number of the cultured embryos increased 9 in number and the mean protein content made the increment of 1.4 mg. Inner ear of the cultured embryos showed nearly as good development as that of 13.0 day embryos.
