210 THE PHYSIOLOGY OF REPRODUCTION 



On Gametic Selection and the Conditions Favourable for the 

 jOccuerence op Fertilisation 



It is a well-known fact in biology that, as a general rule, 

 conjugation occurs most readily between gametes belonging to the 

 same kind of organism. There are innumerable cases, however, in' 

 which the. spermatozoa of one species are capable of fertilising th0 

 ova of another, and so initiating development. The resulting 

 embryo in such cases may grow into a mature hybrid offspring which 

 is> not infrequently sterile (a fact which will be referred to again 

 later), or, on the other hand, owing to some mutual incompatibility 

 in the respective modes of growth inherited from the two parent 

 forms, the embryo may survive for a short time and then perish. 



Cross-fertilisation ' can usually be induced most easily among 

 closely related species or among varieties belonging to the same 

 species. Thus, the different varieties ol the frog, Bana fusda, 

 intercross as readily with one another as each variety fertilises its 

 own pva. On the other hand, the gametes of two species as widely 

 separate as the frog, Banafusca, and the salamander, Triton alpestris, 

 have been known to conjugate, but the fertilised eggs so produced 

 divided irregularly and consequently failed to develop.^ In some 

 cases {e.g. the two species of frogs, B. fusea and B. arvcdis) cross- 

 fertilisation can take place in one direction, but not in the reverse. 

 Pfiiiger 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-fertilisation than those with large stout heads.^ This 

 explanation, while seeming to account for certain individual 

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



Bataillon ^ has described experiments in which he fertilised 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 conjugating, so that the 

 chromatin of the fertilised ova was derived entirely from the female 

 pronucleus. The experiments, therefore, afford additional proof that 

 spermatozoa in conjugating with ova perform a function altogether 

 apart from amphimixis (or ther introduction of fresh chromatin 

 substance as a source of variation), and that this function is the 

 initiation of development. 



' Pfliiger, "Die Bastardzeugung bei den Batrachiern," Pflilger's Archiv, 

 vol. xxix., 1882. 



^ Pfliiger and Smith, " Untersuchungen iiber Bastardierung der Anuren 

 Batrachier, etc.," Pflugei's Archiv, vol. xxxii., 1883. 



^ Bataillon, "Impregnation et F6condation," G. R. de I' Acad, des Sciences, 

 vol. cxlii., 1906. 



