ON THE KXPP:RIMENTAL hybridization of ECHINOIDS. 299 



this character seems to be inherited through the maternal parent, although it is not 

 so strongly developed in E. acutus % X E. esculentus $ as in the pure E. acutus 

 (text-figs. 4 and 5). 



Let us take the arched and straight character of the body-rods. In E. esculentus 

 they are strongly arched, in E. acutus slightly so, and in E. miliaris straight. In 

 the cross E. esculentus % X E. miliaris $ , about 15 per cent, show a slight arching 

 and the rest are straight. In the reciprocal all are straight. Thus the straight form 

 seems to be dominant over the arched, but more completely so v?hen inherited 

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



In E. miliaris $ X E. acutus S the majority of tlie hybrids show a slightly arched 

 form, while in the reciprocal the rods are straight. Thus the straight form is again 

 dominant over the arched, but in this case the dominance is more complete when 

 coming through the male parent (text-figs. 8 and 9). 



Finally, in the cross E. esculentus ^ X E. acutus $ , about 70 per cent, are 

 straight and the remainder are slightly arched, while in the reciprocal about 65 per 

 cent, are straight and the remaining 35 per cent, slightly arched. This is remarkable, 

 since the one parent has a strongly arched form and the other is slightly arched, but 

 neither has the straight type which comes out in the hybrids (text-figs. 4 and 5). 



It is obvious from what we have already said that the characters of the larval 

 skeleton in Echinus are not sufiiciently definite to give clear results in hybridization 

 experiments within this genus. In the end it seems always to remain a matter of 

 personal opinion which parent the hybrid most resembles in the characters of its 

 larval skeleton. We have stated the facts above at some considerable length for what 

 they may be worth, but necessarily there must be a wide difference of opinion as to 

 their value and significance. 



In all the hybrids the development of the larval skeleton is much slower than in 

 the pure-bred forms, and therefore it shows no variation from the rule drawn 

 attention to in Section 7. Furthermore, with regard to the larval skeleton, 

 attention might be drawn to another point of soine importance. We have never 

 been able to distinguish any difference similar to that described by Vernon (98), 

 DoNCASTER (21), and Herbst (39), in the inheritance of the characters of the 

 larval skeleton, which these authors attribute to seasonal or temperature conditions. 

 That is, there is no appreciable difference in the characters of a cross if made at 

 the beginning or end of the loreeding season. Vernon, crossing Strongylocentrotus 

 and SphcBrechinus at Naples, found that the characters of the resulting hybrids varied 

 with the time of year at which the cross was made. If the cross were made in the 

 spring, then the resulting hybrids resembled Strongylocentrotus, while, if the same 

 cross was made in mid-summer, then they no longer resembled Strongylocentrotus, 

 but approached more to the Sphm-echinus type. No such condition holds with 

 regard to our crosses at Plymouth. 



The breeding period of E. acutus at Plymouth extends from May to August, 



2 Q 2 



