230 Relations of Hybrid and Parent Forms of EcMnoid Larvce. 



smaller numbers of the ova reached the pluteus stage in the summer 

 than in the winter months. We see therefore that the Strongylocen- 

 trotus ?-Spha3rechinusc? hybrid is only formed at the time when the 

 Strongylocentrotus ova have reached their minimum of maturity ; 

 ivhilst in the case of the reciprocal hybrid, it follows that as the 

 maturity of the Strongylocentrotus sperm increases, it is able to trans- 

 mute first a portion and then the whole of the hybrid larvae from the 

 Sphaerechinus to its own type. In other words, the characteristics of 

 the hybrid offspring depend directly on the relative degrees of 'maturity 

 of the sexual products. 



As a result of the ten experiments made on the cross Echinus ? - 

 Strongylocentrotus <? , it was found that the hybrid larvae were on an 

 average about 8 per cent, larger than the pure parental larval forms, 

 and, moreover, that even more of the cross-fertilised ova developed to 

 plutei than of the directly fertilised ones. The hybrid larvaB were of 

 a variable, but more or less intermediate, type. In the reciprocal 

 cross, on the other hand, only a small proportion of the ova under- 

 went fertilisation, and only about 1 per cent, of them reached the 

 pluteus stage. These plutei were on an average 13'2per cent, smaller 

 than the pure maternal larvae. In structure they were of a Strongy- 

 locentrotus-inteYmedi&te type. 



Hybrids between Sphcerechinus ? and Echinus c? were obtained 

 on only two occasions. The larvae were of a very variable Echinus- 

 intermediate type, and were much dwarfed. In the reciprocal cross, 

 hybrids were also obtained only twice, but then about 60 per cent, of 

 the ova reached the pluteus stage. These hybrids were of the pure 

 Echinus type. On most other occasions a small number of the ova 

 were cross-fertilised, but failed to develop to plutei. 



Hybrid larvae, of the maternal type, were obtained on crossing 

 Arbacia ova with Strongylocentrotus and with Echinus sperm, but 

 with Sphcerechinus sperm only gastrulae resulted. Hybrids were also 

 obtained between Arbacia $ and Strongylocentrotus, Sphcerechinus, 

 and Echinus ? . These hybrids were of the maternal types, but in 

 some of the Sphcerechinus hybrid larvae the anal arm skeletons were 

 similar to those in pure Arbacia larvae. 



Hybrid larvae were obtained between Echinocardium cordatum $ 

 and Strongylocentrotus, Sphcerechinus, Echinus, and Arbacia <$, the 

 larvaa being of the maternal type, but somewhat modified by the 

 nature of the sperm. The hybrids between Echinocardium 

 mediterraneum ? and Strongylocentrotus and Echinus <$ were of an 

 intermediate type ; hence, one is afforded a physiological argument 

 in favour of the specific difference of these two forms, the existence 

 of which has hitherto been considered rather doubtful. 



Hybrid larvae of the maternal type were obtained on crossing 

 Strongylocentrotus ? with Dorooidaris $ , but the reciprocal cross 



