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ABSTRACT
The jelly capsule of amphibian eggs is an impediment to O2 diffusion to the embryo. This effect can be evaluated with a diffusion equation that relates the rate of O2 uptake (Vo2), Krogh's coefficient of diffusion in jelly (Ko2), the geometry of the capsule which is characterized by its inner and outer radii (r. and ro), and the O2 partial pressure difference across the capsule (ΔPo2). Data for Ko2 are presented to enable calculation of the O2 conductance (Go2) of the capsule from its dimensions. As Vo2 increases during embryonic development, Go2 also increases by swelling of the capsule due to osmotic absorption of water into the perivitelline space; thus, changes in ΔPo2 are minimized. The changes in Go2 appear inexorably linked to the embryonic stage, and cannot be modified during development to adapt to high O2 demand or low ambient Po2. ΔPo2 is very sensitive to changes in ri, but less sensitive to ro. Consequently, failure to absorb enough water into the perivitelline space can result in severe hypoxia, but the existence of a thick jelly coat or boundary layer need not. A preliminary interspecific comparison indicates that Go2 is matched to Vo2 such that the average ΔPo2 near hatching is about 6 kPa. Nevertheless Vo2 may become limited by diffusion through the capsule in late development, and added hypoxia can stimulate hatching. Low availability of O2 to embryos within egg masses is caused mainly by O2 consumption by other eggs in the mass rather than by the existence of jelly. In fact, the jelly aids gas exchange in egg masses by reducing the population density of embryos. Natural selection has resulted in diverse modes of oviposition that avoid the limitations that the egg mass places on O2 delivery to the embryos.