Effects of oxygen supply on development rates of embryos in gelatinous masses were tested with natural masses of three species of opisthobranch gastropods and artificial masses made with embryos of a sea urchin and agarose gel. When diffusive exchange was restricted for embryos in such masses, increased oxygen alone was sufficient to maintain development rates of embryos. Diffusive supply of oxygen is therefore a limiting factor for embryos clustered in gelatinous masses. Development rates were constant over a wide range of experimentally altered oxygen concentrations but retarded with oxygen below about 10% of air saturation. Some opisthobranch embryos exposed to low oxygen concentrations hatched with shorter shells. Delayed hatching and shorter shells at hatching were both associated with a central position in the globose gelatinous mass of one of the opisthobranch species, even in air-saturated and vigorously stirred water. The pH near the centres of the masses was lower when embryos were at more advanced stages; thus the intracapsular stirring caused by cilia of embryos at later stages does not compensate for their greater metabolic rates. The pH in some egg masses was about seven or lower, and this pH affected development in separate experiments. The masses approach limits for removal of wastes, although oxygen becomes limiting for rates of development before accumulation of wastes becomes limiting. Embryos of opisthobranchs that are normally aggregated and embryos of sea urchins that are normally dispersed were similarly tolerant of low oxygen concentrations and low pH. In respect to tolerance of hypoxia and accumulating wastes, embryos in many clades are preadapted for aggregation into masses, but there are limits on the size of the aggregation. Egg masses are commonly at those limits.