CHAPTER III 



EXPERIMENTS IN CROSS-FERTILIZATION 



A number of attempts have been made to fertilize the eggs 

 of one species of frog with the spermatozoa of another species. 

 Rusconi experimented in 1840 with the toad ( $ ) and the green 

 water-frog, Rana esculenta (? ). Lataste in 1878 attempted to 

 cross-fertilize the eggs of different species of urodeles with 

 Pelobates fuscus and P. cultripes. 



Experiments of Pfluger and of Born 



The most extensive and important work is that of Pfluger ('82) 

 and of Born ('83). These investigators have made a large 

 number of experiments in crossing different races and species 

 of Anura. When the sperm of Rana fusca was placed with 

 the eggs of Bufo vulgaris, the eggs segmented and developed 

 as far as the " morula " stage, and then without exception died. 1 

 Conversely, when the sperm from B. vulgaris was used with the 

 eggs of R. fusca, no result followed, not even the segmentation 

 of the egg (except in one experiment where two eggs out of one 

 hundred divided irregularly). Eggs of R. fusca placed in a 

 water-extract of the testes of R. esculenta remained unfertil- 

 ized. But eggs of R. esculenta placed with the sperm of R. 

 fusca developed regularly, with few exceptions, as far as the 

 blastula stage, and then died. Crossing various species of Tri- 

 tons gave no results. But eggs of Rana fusca were acted upon 

 by the sperm of Triton alpestris and Triton tseniatus, inasmuch 

 as they began to show irregular cleavage-lines. Later they 

 died. The reverse cross gave no result. 



Rana fusca and Rana arvalis are very similar in appearance, 

 but are apparently separate species. Cross-fertilization was 



1 Pfluger ('82). 

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