The Development of the Otoliths of the Mouse

T H E R E is still little known about the formation of the mammalian otolith. Work carried out on this problem many years ago (Nishio, 1926) left many questions unanswered. In the meantime considerable knowledge has been gained of the development of other calcined structures, namely, bones and teeth, and the present paper consists of a reinvestigation of the development of the otoliths of the house mouse (Mus musculus L.) in the light of this new knowledge of calcification. The normal fully formed otoliths of the mouse consist of a flat mass of small prismatic calcareous crystals in an organic matrix. The calcium salts of the otoliths of all vertebrates are present as calcium carbonate in the form of aragonite. Hastings (1935), working with the salamander, Amblystoma tigrinum, found in addition 15 -9 per cent, of calcium phosphate in an apatite form. The organic matrix is said to consist of protein. It forms an intimate part of each crystal and fills the spaces between them. According to Tenaglia (1925) it is of a semi-solid consistency and imparts a definite shape to the otolith. Embedded in it are the tips of the sensory hairs of the macula which the otolith overlies. An otolith is present in the sacculus and in the utriculus of each ear. Questions for consideration in a study of the development of the otoliths include the following: which part of the wall of the labyrinth secretes the matrix, what is the mechanism of deposition of calcium salts on the matrix, and why are they deposited only on the matrix? There might be a local secretion of calcium salts by the labyrinth wall, a local absorption of water by the wall, or the matrix itself might actively take part in calcification. Nishio (1926) studied the formation of the otoliths in various vertebrate groups and reviewed earlier work. Previous authors had generally reported droplets emanating from the surface of the macula and had considered these to be otolith material. Nishio, however, interpreted such droplets as fixation artifacts, and