I shall now describe the results obtained with a few typical litters. A litter born of Nos. 151 and 152 gave uniformly negative results in all auditory tests up to the fourteenth day. On that day the ears were open, and the following observations were recorded. The five individuals of the litter, four females and one male, were taken from the nest one at a time at 7 A.M. and placed on a piece of paper in the bright sunlight. The warmth of the sun soon quieted them so that auditory tests could be made to advantage. As soon as an individual had become perfectly still, the Galton whistle was held at a distance of about four inches from its head in such a position that it could not be seen nor the currents of air caused by it felt, and suddenly blown. Each of the five mice responded to the first few repetitions of this stimulus by movements of the ears, twitchings of the body, and jerky movements of the legs. The most violent reactions resulted when the individual was lying on its back with its legs extended free in the air. Under such circumstances the four legs were often drawn together suddenly when the whistle was sounded. Similar responses were obtained with the lip sound already mentioned. Two other observers saw these experiments, and they agreed that there can be no doubt that the mice responded to the sound. The sounds which were effective lay between 5000 and 10,000 complete vibrations. On the fifteenth day the eyes were just beginning to open. Three of the mice responded definitely to the sounds, but the other two slightly, if at all. On the sixteenth day they were all too persistently active for satisfactory auditory tests, and on the seventeenth, although they were tested repeatedly under what appeared to be favorable conditions, no signs of sensitiveness were noted. Although I continued to test this litter, at intervals of three or four days, for two weeks longer, I did not once observe a response to sound. This was the first litter with which I obtained perfectly definite, clear- cut responses to sounds. That the reactive ability had not been present earlier than the fourteenth day I am confident, for I had conducted the tests in precisely the same manner daily up to the time of the appearance of the reactions. To argue that the mice heard before the fourteenth day, but were unable to react because the proper motor mechanism had not developed sufficiently would be short-sighted, for if the response depended upon the development of such a mechanism, it is not likely that it would disappear so quickly. I am therefore satisfied that these reactions indicate hearing.