106 DIFFERENTIATION III 



through the ovum, never through the spermatozoon. The 

 later results obtained by these authors and by Debaisieux 

 suggest that, under certain conditions at least, the posterior 

 epaulettes are dominant, the green pigment recessive, and 

 a similar alternate inheritance of either a paternal or 

 a maternal character, but in any case in its entirety, was 

 reported by Loeb, King, and Moore in the hybrid Strongylo- 

 centrotus franciscanus a* x Strongylocentrotus purpura- 

 tus ? . So Tennent has found that in the cross Toxopneustes 

 variegatus x Hipponoe esculenta the characters of the latter 

 are always dominant in the hybrid, whether it is used as 

 the o* or ? parent. The dominance is not, however, absolute, 

 since only about 70 % of the plutei have the Hipponoe struc- 

 ture (Sphaerechinus type, while Toxopneustes has a larva like 

 that of Echinus), and even then it is not pure. 



Tennent makes the interesting observation that the domi- 

 nance can be altered by decreasing the alkalinity of the sea- 

 water. Under these conditions the percentage of hybrids of 

 the Hipponoe type sinks to about 20 %. Cytological investi- 

 gation has shown that while in one cross (Hipponoe ? x 

 Toxopneustes a*) there is some elimination of chromosomes, 

 though whether paternal or maternal is not known, there is 

 no such elimination in the reciprocal cross. 



Baltzer, who has studied the various crosses of Arbacia 

 pustulosa, Strongylocentrotus lividus, Echinus esculentus, and 

 Sphaerechinus granularis, finds that elimination of chromo- 

 somes occurs in certain cases only, and has further suc- 

 ceeded in identifying the eliminated chromosomes as paternal, 

 and in showing that in these cases the skeleton of the hybrid 

 pluteus is of the maternal type, whereas otherwise it is inter- 

 mediate. 



Thus the cross Sphaerechinus ? x Strongylocentrotus a* 

 gives plutei with intermediate skeleton, as we know, but the 

 plutei reared from the reciprocal cross are of the pure Stron- 

 gylocentrotus type. The cytological examination shows that 

 fertilization is normal up to the metaphase, but that then 

 certain chromosomes begin to lag behind, and so are cast out 

 instead of entering into the daughter nuclei. This elimina- 



