CHAPTER XI 
BEHAVIOR OF THE GERM-CELLS IN CROSS-FERTILIZATION 
In the preceding chapters the characters of the hybrids re- 
sulting from cross-fertilization have been considered. The 
present chapter will deal with the behavior of the germ-cells 
themselves when cross-fertilization is attempted. In certain 
respects this topic covers a wider field than the preceding, since 
there are many more species in which the eggs may be*entered 
by the spermatozoa of other species (and the early development 
take place) than of those that produce adult hybrids. 
Experiments with Amphibia 
Different species of European frogs have been frequently 
utilized in crossing experiments. The most important results 
are those of Rusconi (1840), Lataste (1878), Pfliiger (1882), 
Born (1883), and Heron-Royer (1883). 
It has been found in a number of different forms that the 
spermatozoa of one species will enter the eggs of other species 
and start the development. The egg may cleave, generally quite 
irregularly, but later stages than this may not develop. In some 
combinations the early, or even the later, gastrula stages may 
develop, but the embryos perish without going farther. Finally, 
in a few cases tadpoles, often having a weak constitution, may 
be formed. Thus the two closely similar species, Rana fusca (¢) 
and Rana arvalis (?), cross readily, and tadpoles have been reared 
as far as the frog stage. The reverse cross gave no results. 
Bufo variabilis and Bufo cinereus also cross, and toads may be 
produced. The different races of Rana fusca intercross as 
readily with each other, as each race fertilizes its own eggs. 
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