SPECIFICITY IN FERTILIZATION 201 



ity, as is usual in hybrid combinations. The success 

 of fertilization seems to be entirely unrelated to system- 

 atic relationship. This may be illustrated by the usual 

 difference in the success of reciprocal fertilization; 

 even in species of the same genus a cross-fertilization 

 may succeed one way and fail entirely in the reciprocal. 

 Thus Pfluger reports that the eggs of Rana esculenta 

 fertilize readily with the sperm of R. fusca, the eggs 

 dying in the blastula stage; but eggs of R. fusca 

 can never be fertilized with the sperm of R. esculenta. 

 On the other hand Rana esculenta and Rana aroalis 

 fertilize reciprocally. The eggs of Rana fusca could 

 not be fertilized with the sperm of any other 

 anuran (Pfluger), and the same is true of the eggs of 

 Pelodytes (Bataillon); but the eggs of the latter can 

 be caused to develop by the sperm of Triton alpestris 

 belonging to the order Urodela. 



Born points out that a higher concentration of 

 sperm is usually required for cross-fertilizations than 

 for straight species fertilizations. He distinguishes 

 three kinds of behavior of the gametes in the cross- 

 fertilizations: (i) No reaction; examples: Rana arvalis 

 $ X Rana fusca $ ; Bombinator igneus and Rana escu- 

 lenta reciprocal; Pelodytes $ X Rana aroalis $. (2) In 

 a second group fertilization is apparently monospermic 

 and normal; examples: Rana esculenta X Rana arvalis 

 reciprocal, Rana fusca $ X Bufo cinereus $. (3) In a 

 third group of cases polyspermy is the rule, followed 

 by early death of the eggs; examples: Bufo cinereus 

 $ X Bufo vulgaris ?, Pelodytes $ X Rana esculenta $. 



Unfortunately this material has not been studied 

 cytologically in any systematic way. Born believes 



