20 Hyhridizntion of Echinoids. 



tokonothi may, by accident, be sterile, their sterility, however, not 

 being due to a lack of ability to produce mature germ-cells. 



Successful cross-activation may be followed by the fusion of the 

 germ nuclei, but because of a partial developmental block some 

 paternal chromosomes may be eliminated. Partial hybrids will 

 be the result. Judged by the standard of full capacity for inheritance 

 and development, and tested by cytological examination, even 

 fertile partial hybrids fail to meet the fertilization test. 



Finally, successful cross-activation may be followed by internal 

 block to the fusion of the germ nuclei, and the paternal chromatin 

 may be eliminated, but developmental block may fail to intervene 

 to prevent development. The result of this activation will be the 

 production of false hybrids. These hybrids, again, although they 

 may be fertile, can not be said to have developed from fertilized eggs, 

 since they have not the full capacity for development and inheri- 

 tance nor have all of the chromosomes been retained. 



