11 



EFFECTS OF MIXING DIVERSE ORGANISMS 



HYBRIDIZATION 



I, Incompatibility of Paternal and Maternal Germ Cells 



in Development 



In the chapters up to this point we have described what 

 may be considered the normal operation of the genetic sys- 

 tem and the results of this operation. We now examine 

 certain features that may be called abnormal, resulting 

 from the crossing of very dissimilar organisms. 



Union of germ cells to produce offspring commonly oc- 

 curs only in the case of closely related organisms having 

 germ cells and genetic systems that are closely alike. Very 

 diverse organisms, such as mollusks and echinoderms, or 

 starfish and sea urchins, differing greatly in their chromo- 

 somes or in other features of the germ cells, commonly do 

 not cross. When the sperms of one such type are brought 

 into contact with the ova of another, the sperms do not 

 enter the ova; there is no fertilization. 



A method of inducing cross-fertilization between cer- 

 tain very diverse marine organisms was discovered by 

 J. Loeb.^ By putting certain chemicals in the sea water, he 

 found that the sperms from one type could be caused to 

 enter the ova of very different types, bringing about devel- 

 opment. In this way he induced fertilization of the ova of 

 the sea urchin by sperms of starfish, holothurians, and even 

 mollusks and annelids. Later such work was greatly ex- 

 tended; many other distant crosses were made. 



It turned out that in such crosses between very diverse 



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