A block to cross-fertilization located in the egg jelly of the frog Rana clamitans.

The egg of Rana clamitans enrobed in its native jelly was not fertilized by sperm of R. pipiens. However, when R. clamitans eggs were enrobed by R. pipiens jelly, they were fertilized by R. pipiens sperm. Male pronuclei were found in the eggs, and most of the resulting embryos were diploid. The embryos gastrulated, but most arrested at midto late gastrula stages. Some begun neurulation, but none survived longer than 4 days. When R. clamitans eggs in R. pipiens jelly were fertilized by R. clamitans sperm, the embryos developed normally except that they failed to hatch. In the reciprocal experiment, R. pipiens eggs were enrobed in R. clamitans jelly. The eggs were not fertilized by R. pipiens sperm but were fertilized by R. clamitans sperm. Therefore, the R. clamitans jelly plays a major role in preventing fertilization by sperm of other species. The if?, clamitans jelly's block to cross-fertilization was not a block to sperm migration. Further, the R. clamitans jelly contained factors which permitted R. pipiens sperm to fertilize dejellied R. pipiens eggs. Dejellied eggs are usually not fertilizable in the absence of jelly factors.