Abstract
This forms the second report on developmental rate in hybrid frogs. The purpose of these experiments, in hybridizing species that differ in rate of development, is to determine the earliest effect of the sperm on this particular character. Previously such a study has been made on hybrids between Rana pipiens and Rana palustris. 1 Both hybrids, pipiens ♀ × palustris ♂ and palustris ♀ × pipiens ♂ developed at the maternal rate during early stages. The first unmistakable deviation from the maternal rate appeared in the neural plate stage in both hybrids. Additional evidence on these points is derived from the following crosses.
The material used in these experiments is from the following sources: Rana pipiens Schreber from dealers in the Lake Champlain area of Vermont and Rana sphenocephala (Cope) from dealers in southern Florida. Although listed as separate species by many taxonomists the two are closely related and are thought best included under one name by Kellogg. 2 Regardless of their true taxonomic status the two frogs form viable hybrids and differ in rate of growth to such an extent that it is possible to analyze the developmental rate of their hybrids. At 15°C the rate of both species is identical. At higher temperatures sphenocephala develops more rapidly and below 15° less rapidly than pipiens. The egg diameter in the sphenocephala used in these experiments averaged 1.3 mm and in the pipiens 1.8 mm. Whereas the upper limit for normal development in pipiens is 28° or 29°, the Florida species can tolerate temperatures as high as 34.5°.
The first experiment was begun Jan. 20, 1939. Gametes of both male and female pipiens and sphenocephala were used, the eggs being secured by pituitary injections and sperm by cutting the testes in 0.1 amphibian Ringer's.
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