Studies in Heredity. 



241 



eggs of Echinus with the sperm of Echinocardium, which were purely 

 maternal in character. I may add that, in addition to repeating the 

 experiments which I had performed two years before, I also crossed the 

 species Echinus miliaris with Echinocardium cordatum, but this species gave 

 practically the same results as Echinus esculentus, and, except where other- 

 wise stated, the remarks in this paper will apply to the cross between 

 Echinus esculcntus and Echinocardium cordatum. 



Every possible precaution was adopted to prevent sperm infection The 

 outsides of all the urchins used were washed in a copious stream of fresh 

 water before they were opened, in order to destroy any adhering sperm ; all 

 the instruments employed were likewise washed in fresh water, and the 

 fertilisation of the eggs was effected in sea-water, which had been thoroughly 

 sterilised by previous heating to 80° C. 



One fact which forced itself prominently on one's notice this year was the 

 individual idiosyncrasy of urchins with regard to the capacity for being 

 crossed. Sometimes two urchins when crossed, although to all appearance 

 ripe, would not yield a single hybrid larva — yet the eggs developed 

 rapidly when fertilised with their own sperm. Another time, females 

 apparently only half ripe yielded eggs which when fertilised with the foreign 

 sperm produced a certain proportion of hybrid larvae. 



In general it may be said that the eggs of Echinocardium cordatum in 1913 

 were much more resistant to the action of the sperm of Echinus esculcntus 

 than they were in 1911. In some cases these eggs refused to develop at all, 

 but broke up by the process of cytolysis into a multitude of spherules, just as 

 I had described the eggs of Echinus esculentus doing when fertilised with the 

 sperm of Echinocardium cordatum (fig. 1). Indeed, in all cases this may be 



Fig. 1. — Egg of Echinocardium cordatum, fertilised with the sperm of Echinus esculentus 



breaking up into globules. 



said to have been the case of most, but in many cases a certain proportion of 

 hybrid larvae were produced which showed in every detail the features which 

 I had described in the hybrids obtained in 1911, and I shall not therefore 



