FERTILISATION 203 



failed to develop.^ In some cases (e.g. the two species of frogs, 

 R. fusca and R. arvalis) cross-fertilisation can take place in one 

 direction, but not in the reverse. Pfliiger explained this result 

 by supposing it to be due to peculiarities in the shape or structure 

 of the spermatozoa, those which have the thinnest or most 

 pointed heads being described as more successful in inducing 

 cross-fertihsation than those with large stout heads. ^ This 

 explanation, while seeming to account for certain individual 

 instances, cannot be applied to all cases of cross-sterihty. 



Bataillon ^ has described experiments in which he fertihsed 

 the eggs of Pelodytes and Bufo with the spermatozoa of Triton 

 alpestris, and obtained some degree of success, for the eggs in 

 each case underwent an irregular segmentation before they 

 perished. The spermatozoa underwent degeneration after con- 

 jugating, so that the chromatin of the fertihsed ova was derived 

 entirely from the female pronucleus. The experiments, there- 

 fore, afford additional proof that spermatozoa in conjugating 

 with ova perform a function altogether apart from amphimixis 

 (or the introduction of fresh chromatin substance as a source 

 of variation), and that this function is the initiation of de- 

 velopment. 



Among the Mammaha, as is well known, cross-fertilisation 

 between nearly allied species commonly occurs. The resulting 

 hybrid may be either sterile (e.g. the mule) or fertile (e.g. the 

 hybrid offspring of the bull and American bison). There is 

 no evidence that more widely separated species of Mammals 

 can be induced to have hybrid offspring. Spallanzani,* by 

 artificially inseminating an oestrous bitch with the spermatozoa 

 of a cat, attempted such an experiment, but without a positive 

 result. 



A number of investigators have effected cross-fertihsation 

 between various kinds of Bchinoderms. Vernon,^ who experi- 



1 Pfliiger, "Die BastardzeuguDg bei den Batraohiern,'' Pfiiigers Arch., 

 vol. xxix., 1882. 



' Pfluger and Smith, " Untersuchnngen iiber Bastardierung der Anuren 

 Batrachier," &c., Pfliigir's Archiv, vol. xxxii., 1883. 



" Bataillon, " Impregnation et Fecondation," C. R. de I'Acad. des Sciences, 

 vol. cxlii., 1906. 



• Spallanzani, Dissertations, English Translation, vol. ii., London, 1784. 



5 Vernon, "The Belation between the Hybrid and Parent Forms of 

 Echinoid Larvse," Phil. Trans. B., vol. oxc, 1898. 



