SPECIFICITY IN FERTILIZATION 201 



ity, as is usual in hybrid c()ml)inations. The success 

 of fertiUzation seems to be entirely unrehited 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-fertiHzation 

 may succeed one way and fail entirely in the reciprocal. 

 Thus Pfluger reports that the eggs of Rana esc id cut a 

 fertilize readily with the sperm of R. fuse a, the eggs 

 dying in the blastula stage; but eggs of R. fuse a 

 can never be fertiHzed with the sperm of R. eseuleuta. 

 On the other hand Rana eseuleuta and Rana arvalis 

 fertilize reciprocally. The eggs of Rana fuse a coukl 

 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 fusea ?; Bombinator igneus and Rana eseu- 

 leuta reciprocal; Pelodytes 6 X Rana arvalis 9. (2) In 

 a second group fertilization is apparently monospermic 

 and normal; examples: Rajia eseuleuta X Rana arvalis 

 reciprocal, Rana fusea ^ X Bufo eiuereus ?. (3) In a 

 third group of cases polyspermy is the rule, followed 

 by early death of the eggs; examples: Bufo eiuereus 

 $ X Bufo vulgaris $, Pelodytes .T X Rana eseuleuta 9. 



Unfortunately this material has not been studied 

 cytologically in any systematic way. Horn believes 



