ELEVEN FIGURESThe first study of developmental changes in chemical composition of sea urchin embryos was carried out by Ephrussi and Rapkine in 1928. Relative to the unfertilized eggs of Paracentrotus lividus their figures show that a loss of 4.4% total nitrogen ( N ) , 3470 carbohydrate and 24% lipid occurred in development to the pluteus stage. Hayes ('34) found the total N of Echinometra lucunter to decrease 24% and the free amino N to decrease 11% in development from the 4-hour blastula to the late prism stage. On the other hand, Horowitz ('39) found the total N of the gephyrean worm Urechis caupo to remain practically constant in development to the 24-hour trochophore although an 8.4% decrease occurred by the 30th hour. An important feature of EIorowitz's work is that his assay results were related to one another by counting the number of embryos used, rather than by determinations of dry weight, centrifuged volume or total N (see also Hayes, '38).