ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. '2 97 



S. franciscanus $ X -S. ptirpuratus $ . Moore (64), repeating this cross the 

 following year, experienced the same difficulty ; the larvae were, with a few 

 exceptions, pathological. 



In our preliminary paper we said that the evidence from our crosses seemed to 

 support the contentions of LoEB, King, and Moore, but that in our crosses the female 

 seemed to have a stronger influence than the male. In re-examining the matter this 

 year we have come to the conclusion that the evidence is not really strong enough to 

 warrant our maintaining this view. An examination of text-figs. 1-9 will make this 

 plain. These figures will show, however, how exceedingly difficult it is to draw 

 definite conclusions from the skeletons of the hybrids, owing to the fact that in any 

 particular cross the individuals vary so widely among themselves. 



We give a series of drawings of the aboral ends of the body-rods in E. esculentus, 

 E. acutus, and E. miliaris, and all the hybrid crosses between these species, about 

 40 examples being shown of each (text-figs. 1-9). It will be seen from these figures 

 that the aboral ends of the body-rods in E. esculentus may be described as slender, 

 rather spinous, and arched. E. acutus is slightly clubbed, very spinous, and slightly 

 arched. E. miliaris is clubbed, smooth, and straight. Thus we make out three 

 pairs of characters : namely, clubbed and slender, spinous and smooth, and arched and 

 straight (text-figs. 1-3). 



In the fii-st place, E. acutus has a skeleton which is slightly clubbed and slightly 

 arched (see text-fig. 2). Since clubbedness and archedness both occur in the same 

 form, they cannot be an allelomorphic pair. Rather, clubbedness seems to be opposed 

 to slenderness and archedness to straightness. 



Now E. acutus is very spinous, while E. miliaris is smooth. Both reciprocal 

 crosses between these two forms give spinous skeletons (text-figs. 8 and 9), so that 

 spinousness seems to be dominant to smoothness. Again, E. esculenttis is slightly 

 spinous while E. miliaris is smooth. Reciprocal crosses between these species are, 

 however, both smooth, so that here smoothness is dominant over spinousness (text- 

 figs. 6 and 7). Loeb, King, and Moore found that for Stongylocentrotus franciscanus 

 and S. purpurutus spinousness was dominant over smoothness. From the foregoing, 

 however, it- is plain that this cannot be taken as a general rule : difierent species 

 seem to behave differently. 



With regard to the clubbed and slender character, E. miliaris is clubbed, E. acutus 

 slightly so, and E. esculentus slender, so that it is crosses between E. esculentus and 

 the other forms which must be considered. In E. miliaris % X E. esculentus $ , the 

 hybrids have the clubbed form, while in the reciprocal about 75 per cent, are slightly 

 clubbed and 25 per cent, slender. In crossing E. esculentus and E. miliaris 

 clubbedness seems to be dominant to slenderness, but more so when inherited 

 through the female parent (text-figs. 6 and 7). 



In the cross E. acutus ^ X E. esculentus $ , the rods are slightly clubbed, while in 

 the reciprocal they are slender. In crossing E. acutus and E. esculentus, therefore, 



vol. cciv. — B. 2 Q 



