174 Experimental Zoology 



The other extreme is found where the eggs of Rana fusca are 

 fertihzed by the eggs of the salamander, Triton, and divide 

 irregularly, but go no farther. 



Pfluger concluded from his results that cross-fertiHzation de- 

 pends less on the similarity of the adults than on the pecuharities 

 of the spermatozoa. Thus the spermatozoa of Rana fusca and 

 Rana arvaHs are very different, and while cross-fertihzation 

 takes place in one direction it does not in the other. On the other 

 hand the spermatozoa of Bufo cine reus and B. variabilis are much 

 ahke, and reciprocal cross-fertilization is successful. In support 

 of his view Pfluger points out that the spermatozoa are most suc- 

 cessful in crossing that have the thinnest or most pointed heads. 

 Furthermore those eggs are most easily crossed that belong to 

 species whose spermatozoa have the largest heads, because, being 

 as it were so constructed as to admit their own large-headed sper- 

 matozoa, they do not exclude spermatozoa of smaller size. This 

 view of the matter may explain the power of certain kinds of 

 spermatozoa to enter the eggs of other species, but it does not ex- 

 plain why, after entering, certain combinations develop normally 

 and others scarcely at all. 



The conditions for normal development appear to be most 

 readily fulfilled when the two species are Hke each other in struc- 

 ture, which usually, though not invariably, means consanguinity. 

 Pfluger found that the eggs of the frog have the greatest power 

 of being "cross-fertilized at the height of the breeding season. 

 Certain experiments that I have made on other forms indicate 

 that this result may be due not so much to the eggs as to the 

 greater mobility of the spermatozoa at this time. Hertwig has 

 questioned Pfliiger's conclusion, basing his objection on the evi- 

 dence derived from some experiments that he carried out on the 

 eggs of the sea urchin. He found that eggs could be more easily 

 crossed when overripe or stale, as when they have stood for some 

 hours in sea water, or after they have been injured by poisons. 

 It has been showTi more recently by Vernon that while in a few 

 cases {i.e. in some species) more eggs can be cross-fertiHzed if 

 they have stood twelve to twenty- four hours in sea water, yet in 



