The 45 Ca turnover in the membranous labyrinth of chick embryos during development By MARIO

As is well known, otoliths are composed in most vertebrates of calcium salts which are in the form of carbonates. Yet, Hastings (1935) in Amblystoma tigrinum and Carlstrom & Engstrom (1955) in Petromyzon found otoliths in these species to be made of calcium phosphate. From a crystallographic point of view the researches of these two authors have provided evidence that the otoliths of birds, of cartilaginous fishes and even those of mammals (contrary to the generally accepted opinion) are made of calcite, while those of amphibians and of bony fishes are made of aragonite. In foetuses of fishes (Acanthias vulgaris) Vilstrup (1951) records the presence of 'vacuoles' containing calcite crystals in the epithelium of the endolymphatic duct and on the surface of the sacculus. He thinks he can show a triple origin of otoliths in Elasmobranchs: (1) exogenous otoliths made of quartz crystals coming from sand grains, which, according to him, reach the labyrinth through the endolymphatic duct which, in the Elasmobranchs, opens freely on the surface of the skin over the dorsum cranii; (2) otoliths produced in situ at the level of maculae; and (3) otoliths coming from the endolymphatic duct which on reaching the level of the maculae become embedded within a gelatinous substance. This last process takes place only after birth of the animal. In embryos and new-born mammals (mice, cats) Lyon (1955) found a gelatinous layer with an organic matrix and some inorganic salts in the otolithic membrane. In the early stages of development she found a PAS-positive precipitate (organic matrix) within the sacculus and the utriculus. This precipitate shows a birefringence due to the presence of calcium salts. According to this author, the first appearance of calcium salts would be simultaneous with that of the matrix. Yet Lyon (1955) did not determine exactly the ways in which the calcification of otoliths takes place. As for the place of origin of the material concerned with the formation of otoliths, part of it is thought to be produced