174 Experimental Zoology 
The other extreme is found where the eggs of Rana fusca are 
fertilized by the eggs of the salamander, Triton, and divide 
irregularly, but go no farther. 
Pfliiger concluded from his results that cross-fertilization de- 
pends less on the similarity of the adults than on the peculiarities 
of the spermatozoa. Thus the spermatozoa of Rana fusca and 
Rana arvalis are very different, and while cross-fertilization 
takes place in one direction it does not in the other. On the other 
hand the spermatozoa of Bufo cinereus and B. variabilis are much 
alike, and reciprocal cross-fertilization is successful. In support 
of his view Pfliiger 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 like each other in struc- 
ture, which usually, though not invariably, means consanguinity. 
Pfliiger 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 shown more recently by Vernon that while in a few 
cases (7.e. in some species) more eggs can be cross-fertilized if 
they have stood twelve to twenty-four hours in sea water, yet in 
