SKELETON FORMATION OF SEA URCHIN LARVAE. I. EFFECT OF CA CONCENTRATION OF THE MEDIUM

Abstract

The threshold Ca concentration for the growth of the larval skeleton is lower than that which permits the normal development of sea urchin larvae. The time of the beginning of skeleton formation is retarded by a decrease in Ca concentration. Modification of the Ca concentration of the medium influences the form of the skeleton of sea urchin larvae. In larvae transferred to very dilute Ca before the time of skeleton formation, a mass of unorganized skeletal substance develops. Larvae in very concentrated Ca at a similarly early stage form several-tri-radiate spicules, but these fail to make a typical skeleton. If the transfer is deferred until the tri-radiate spicules are formed, new skeletal development in the experimental media is continued on the old spicules, and a pluteus skeleton which is similar to the typical one results, although it is much stunted in Ca-low, and slender in Ca-high media. The proportion of the thickness to the length of the skeleton is inversely proportional to the Ca concentration. This is true regardless of the stage of transfer. The elongation of the skeletal rods is inhibited at the extremes of the Ca-series, particularly on the dilute side. The ectoderm of the presumptive arm may differentiate without any skeleton. However, such differentiated ectoderm finally falls off the body or disappears unless the supporting skeleton develops. It is suggested that an organic matrix may be formed before deposition of the mineral components of the skeleton, that this matrix is deformed by changes in the Ca concentration, and that such deformation, in turn, influences the rate of growth and the form of the skeleton.