The distribution of alkaline phosphatase in the skull of the developing trout.

THIS work is part of a comparative study of the histological and cytological distribution of alkaline phosphatase in developing teleost and elasmobranch fishes, made with a view to throwing some light on the mechanisms involved in calcification and ossification. Whereas the adult mammalian skeleton is almost completely ossified, in the lower vertebrates cartilage persists side by side with bone. In elasmobranchs bone does not occur but calcium deposits are formed in the cartilage. Here calcification can be studied independently from ossification. No histochemical study of phosphatase in lower vertebrates has so far been published. There is, however, evidence that phosphatase exists in teleosts as well as elasmobranchs, and is closely similar to the alkaline phosphatase found in ossifying parts of mammals (Bodansky, Bakwin, and Bakwin, 1931; Roche and Bullinger, 1939). The latter authors found a correlation between the degree of ossification and the phosphatase content in different species of fish and at different stages of development in the same species. A study of phosphatase in relation to the growth of scales in elasmobranchs and teleosts leads Roche, Collet, and Mourgue (1940) to conclude that here, as in the higher vertebrates, phosphatase is concerned with rapid osteogenesis. These results suggest that the same biological mechanism is operative in the calcification of the skeleton in fish as in mammals. In view of this a systematic study of the distribution of phosphatase and calcium salts in developing teleost and elasmobranch embryos seemed of interest. This incidentally provides material for the study of phosphatase in the embryo generally. Studies so far undertaken in this direction were confined to young fowl embryos (Moog, 1944) and the heads of rat embryos (Horowitz, 1942). This paper deals only with ossification in a typical teleost. The results obtained with elasmobranch embryos will be published separately. The trout (Salmo spp.) was chosen because it can conveniently be reared in the laboratory. Moreover the development of Salmo has been intensively studied from