Ch. Ill] EXPERIMENTS IN CROSS-FERTILIZATION 27 



here possible (R. fusca, $ , R. arvalis, 9 ). Tadpoles developed 

 from the crossed eggs, and some of these ultimately transformed 

 into frogs. Pfliiger got similar results with the same species, and 

 also found that the reverse cross (LI. fusca, 9 , and R. arvalis, $ ) 

 gave no result. Born found that the eggs of Bufo cinereus 

 could readily be fertilized with the sperm of Bufo variabilis. 

 All the eggs segmented regularly, the larvae left the jelly, and 

 developed into frogs. 



In respect to the closeness of the relation between the species, 

 Born says that we can be quite certain that the two species of 

 Rana arvalis and R. fusca are much more nearly related than the 

 two species of Bufo. The success of cross-fertilizing depends 

 apparently less on the degree of relationship, as shown by the 

 similarity of color and habits, than on the similarity of the male 

 sexual products (Pfliiger). Although R. fusca and R. arvalis 

 seem to be very closely allied species, they have very different 

 spermatozoa ; in fact, the spermatozoa are as different as the 

 spermatozoa of R. fusca and R. esculenta. 1 The two species of 

 toads (Bufo) have very similar spermatozoa, which differ only 

 in size, but this difference is so slight that, were the two kinds 

 mixed together, one could scarcely distinguish between them. 

 It is apparently owing to the difference in form of the sperma- 

 tozoa of the R. fusca and R. arvalis, and to the similarity of the 

 spermatozoa of B. cinereus and B. variabilis that the results are 

 due. 



Pfliiger has made a large number of reciprocal crosses between 

 different races of R. fusca. " The different races are as fertile 

 inter se as are individuals of the same race." Pfliiger concluded, 

 after comparing the results of all of his experiments on cross- 

 fertilization, that in general those spermatozoa are most successful 

 for purposes of cross-fertilization that have the, thinnest and most 

 pointed heads. That in general those eggs are most easily fer- 

 tilized that belong to species having spermatozoa with thick 

 heads. The results, then, he thought, depend largely upon me- 

 chanical conditions ; for where the head is small and pointed, the 

 spermatozoon can bore its way more successfully into the eggs 



1 R. arvalis and R. esculenta have similar sperm. Born and Pfliiger found 

 that the crossed eggs segmented irregularly, and that later the embryos all died. 



