1 92 PROBLEMS OF FERTILIZATION 



encountered as by other authors. But little can be 

 learned, therefore, concerning the kind and degree of 

 the natural specificity of the fertilization reactions in 

 these cases, except what has been stated, viz., that in 

 nine out of the twelve cross-fertilizations hyperalkaline 

 conditions are necessary to permit penetration of the 

 spermatozoon. But, after the cortical specificity is 

 broken down by the hyperalkaline medium, fertiliza- 

 tion proceeds normally up to the metaphase of the 

 first cleavage, when elimination of paternal chromo- 

 somes occurs regularly in certain cases. In other cases 

 elimination of paternal chromatin is postponed until 

 the blastula stage, and in yet others it does not occur 

 at all. But there is only one case in which the recip- 

 rocals behave alike in this respect, viz., in the Echi- 

 nus XStrongylocentr otus crosses, among the six pairs of 

 reciprocals. , 



There is certainly a quantitative specificity in these 

 cases. For a critical examination of the problem of 

 specificity within the class we need a quantitative con- 

 trol of egg and sperm concentration for the specific and 

 the cross-fertilizations which should be reciprocal, and a 

 careful cytological examination of the eggs. In the 

 second place we need also, in all cases in which the 

 gametes are refractory to cross-fertilization, an exper- 

 imental testing of methods for overcoming the diffi- 

 culty. Until this is done we cannot say what the 

 measure of specificity really is. In the third place the 

 viability of the crosses should be thoroughly tested; 

 it has been shown in many cases that development 

 becomes abnormal in the blastula or gastrula and, in 

 other cases, in pluteus stages. But some crosses even 



