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 (9), 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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