SPECIFICITY IN FERTILIZATION 189 



but in this case it is difficult to suggest why it should 

 exist in one reciprocal and not in the other. Gray 

 (1913) found that treatment of fertilized eggs of E. 

 acutus with hypertonic solutions of medium strength 

 caused elimination of some chromosomes, but the 

 phenomenon could not be induced in E. esculentus by 

 similar treatment. He suggests that the chromosome 

 behavior in the reciprocal crosses might be understood 

 on the assumption that the osmotic relations are different 

 in the cross and pure species, owing to different effects, 

 on permeability of the egg, of the foreign and species 

 sperm. This would imply that the eliminated chromo- 

 somes are of maternal origin; however, this cannot be 

 proved in the cross under consideration, and it is known 

 that in wider crosses the eliminated chromosomes are 

 of paternal origin generally (Herbst, Balzer, Tennent). 



2. Interspecific crosses among the echinoderms seem 

 to have been confined to the genus Echinus, and we 

 are therefore unable to make any general statement 

 concerning the possibilities. But wider crosses within 

 the order have been made very frequently. Vernon 

 (1900) alone attempted forty-nine out of a possible 

 fifty-six cross-fertilizations between eight species belong- 

 ing to seven genera of sea urchins; only eleven of these 

 gave no sign of cross-fertilization; of the remainder, 

 nine gave only segmentation stages or blastulae or 

 gastrulae, and twenty-nine lived to the stage of eight- 

 day plutei. 



In Vernon's cross-fertilizations a high sperm con- 

 centration seems generally to have been employed. 

 The percentage of eggs fertilized was nevertheless small 

 as compared with species fertilization; in many cases 



