Effects of fusion in pairs on the development of anurans Rana dalmatina and Rana esculenta.

Of 252 heterospecific, parabiotic pairs of Rana dalmatina and R. esculenta, fused during embryonic life, 87 died because the larva of R. dalmatina died at the beginning of its circulation. Later 89 pairs died when the R. esculenta larva died suddenly at the beginning of its independent feeding. Seventy-one pairs survived at the end of the metamorphic climax. Death in the first case is considered to result from some toxic effect, in the second from an immunological reaction. In the surviving pairs the duration of larval development in the rapidly developing species, i.e., R. dalmatina, was extended from two months to about four months, and in the more slowly developing species, i.e., R. esculenta, was reduced from about five months to about four. These changes in developmental rates are discussed in terms of changes in the concentration of the hormones controlling the rate of development due, in turn, to the parabiosis of embryos with different genetic constitution.