The previous review of theories does not consider the possibility of the specific energy of brain centers, rather than of the peripheral sense mechanism—that is, the possibility that the sensation qualities, sweet, sour, etc., may be due to structural characteristics of brain centers, rather than of sense organs. This is a question which is unsolved for other senses and consequently is not peculiar to taste. The present tendency seems to be toward attributing the different sensation qualities to the sensitivity of the sense organ, or else to divide it between brain center and sense organ. The following facts are considered as indirect evidence of the specific energy of the sensory ends of taste: (1) The distribution of taste sensitivity over the tongue. (2) The effect of certain drugs, e.g., cocaine, which destroys the taste sensations one at a time. This suggests that the sensory ends that have to do with the different sensations are differently affected by the drug. (3) The fact that the same substance as it passes over the surface of the tongue arouses different sensations, e.g., sodium sulphate, which is sweet on the tip and bitter on the back of the tongue. ------------------------------------------------------------------------ CHAPTER IX THE DEVELOPMENT OF TASTE IN THE INDIVIDUAL _Development Before Birth_ THE structures concerned with the taste sensations develop early in the life of the human embryo. As early as the twelfth day the rapid development of the head end of the embryo causes an infolding of the _ectoderm_ (outer layer of the embryo), which later forms the mouth and nasal cavities. And about the twelfth week the mouth structure is fairly complete. It has at this stage become separated from the nasal cavity through the growth of the hard and soft palate. At the twelfth week the tongue is also fairly well developed as a single organ, although originating from three or four separate parts which grow together. The anterior two-thirds of the tongue—that is, all of that portion back to and including the rows of circumvallate papillæ of the adult tongue,—grows from the floor of the original mouth structure. It is in this portion alone that papillæ are to be found. At the end of the fourth week the circumvallate papillæ begin to appear and at the twelfth week the fungiform and filiform papillæ begin to appear.