The distribution of alkaline phosphatase has been well established in several mature tissues and organs (Gomori,'41; Bourne,'43). Similar observations have been recorded for embryonic tissues. Kabat and Furth ('41) have reported phosphatase in the mesenchymal anlagen of the leg and tail of the mouse embryo. Later, Wilmer ('42) observed that the cells of the embryonic chick heart cultivated in vitro contained phosphatase. I na study of the fetal rat, Horowitz ('42) observed that the enzyme was localized in several regions of the head. Moog ('44) in a thorough study of the developing chick has also recorded the presence of alkaline phosphatase in most of the organs and tissues at some stage in their development. Recently, Bevelander and Johnson ('45) completed a study in which the localization of phosphatase was ascertained in the devel6ping tooth.

It occurred to us that it would be of interest to make a study of the distribution of phosphatase in the developing hair and to compare the apparent role of alkaline phosphatase in the development of the tooth and the hair follicle, each derived from ectoderm and mesenchyme. In the one case keratinized hair results, and in the other, two calcified tissues, dentin and enamel.