200 PROBLEMS OF FERTILIZATION 



second cleavages, though it did not occur in the recip- 

 rocal crosses of the same cases. 



Newman could find no relation whatever between 

 success in development and taxonomic relationship, 

 though Moenkhaus is of the contrary opinion. Some 

 of the crosses between species of the same genus exhib- 

 ited much less success in development than other 

 crosses between members of different orders. The eggs 

 of some species (e.g., Fundulus majalis) never hybrid- 

 ize well, while those of others do well with the sperm 

 of all species tried (e.g., Tautoglabrus). Similarly the 

 sperm of some species is better adapted to hybridiza- 

 tion than that of other species. There is a frequent 

 marked difference in the success of reciprocal crosses, 

 but there are notable exceptions. In Newman's opin- 

 ion the factors concerned in success or failure of hybrid 

 development of teleosts are associated with amount, 

 composition, and density of yolk, hardiness or delicacy 

 of the species concerned, and certain mechanical advan- 

 tages or disadvantages. Chemical specificity thus ap- 

 parently does not play a leading role, though I believe 

 it would be a mistake to assume that it is absent. 



c) Amphibia: Hybridization in Amphibia has been 

 studied by Pfluger (1882 and 1883), Born (1883 and 

 1886), and Bataillon (1906, 1909, 1910) among others 

 in a large number of crosses, especially in the order 

 Anura. The hybrid eggs never develop to meta- 

 morphosis, except in the cases of Rana fusca $X 

 Rana arvalis ? and Bufo vulgaris $ X Bufo cinereus $ 

 (Born). The other combinations die at various stages, 

 usually early. With respect to fertilization there is 

 immensely greater success than with respect to viabil- 



