The inner ear of the mouse develops from a thickening of the ectoderm called the otic placode which appears about the eighth day of gestation. The otic placode develops into a completely formed but cytologically undifferentiated inner ear by the 14th day of gestation." The terminal mitoses of most of the cell types which compose the adult inner ear take place on the 12th to the 15th day of embryonic life. 7 The 11th and 12th day otocyst was chosen for this study since the otocyst is in an active stage of cellular proliferation. The following studies are designed to determine the length of the cell generation time and the amount of time in which the cell remains in each portion of the cell generation time. 5-0 A pattern of nuclear migration was observed, and findings similar to those of the neural tube were noted. 2'8-10

PROCEDURE. Fifty pregnant CBA-J/CBA-J mice were used. The gestation time was determined by the vaginal plug method. The morning when the plug was found was called gestation day one. Between 8: 30 and 9: 30 am each pregnant mouse was intraperitoneally injected with six microcuries of tritiated thymidine per gram of body weight. The specific activity of the tritiated thymidine was three curies per millimole. After injection two animals were sacrificed each hour of the 12-hour period. The embryos were removed, fixed in 10 percent acrolein in phosphate buffer, Ph 7.4, for one hour and dehydrated in a 1: 1 (v/v) solution of methyl alcohol and methyl cellusolve for three days. Two embryos from each litter were infiltrated for three days and then imbedded with 90 percent polyethylene glycol, 400 disterate and 10 percent cetyl alcohol by weight. The specimens were oriented in a coronal plane and serially sectioned at five microns on a Leitz rotary microtome. The slides were dipped in a 1: 1 (v/v) solution of