ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 345 



17. Summary of Conclusions. 



1. As the result of extensive investigation of the early larval characters of various 

 crosses between E. esculentus, E. acutus, and E. miliaris, vpe have come to the 

 conclusion that these are too variable to afford any definite evidence of parental 

 influence, and especially is this true vpith regard to the skeleton, heretofore considered 

 the chief index of inheritance. 



2. Kegarding the early larval characters, therefore, as of too variable a nature, we 

 have reared the normal and hybrid crosses to the young urchin stage, in the hope 

 of finding, in the late development, more definite characters for the solution of 

 the question of inheritance. In the presence or absence of the posterior ciliated 

 epaulettes, and of the green pigment masses, we claim that we have found such 

 definite characters, and that these have shown no variation whatever in their 

 inheritance in the pure-bred forms. 



3. During the years 1909-11, the inheritance of these characters in the hybrids 

 was invariably through the maternal parent, the reciprocals of a cross being luilike. 



4. In the season of 1912 their inheritance in the hybrids was different from that of 

 previous years, the offspring of crosses with E. miliaris eggs being paternal, not 

 maternal. This gave a dominance of E. esculentus or E. acutus characters over 

 those of E. miliaris in both reciprocals of a cross alike. 



This rule was not invariable in 1912, a few cultures giving mixed larvae. 



5. Different cultures from the one given fertilization have always given the same 

 type of inheritance. 



6. We have shown that, in the season of 1911, no alteration of the alkalinity of the 

 water affected the inheritance of the skeletal characters, nor those of the late larvae in 

 either the pure forms or the hybrids. 



7. It is suggested that the change of inheritance in the season 1912 was due to 

 some factor affecting the germ cells of E. miliaris, especially the eggs, during the 

 period of their growth and maturation. 



8. The cause was not the relative ripeness of the eggs, as crossing at the beginning, 

 middle, and end of the breeding period of each species gave no difference in 

 inheritance. It was not due to changed conditions at, or after, fertilization, as 

 changed alkalinity of the sea-water and different laboratory temperatures had no 

 influence. That the general physiological condition of the E. miliaris eggs was 

 different in 1912 is indicated by the ill-health of pure cultures of this species, and 

 by the low percentages of fertilization and the alteration in inheritance in crossing 

 with E. miliaris % . 



9. Some evidence has been brought forward to show that the pecuHar temperature 

 conditions of the sea- water at Plymouth in the season 1911-12 may have played a 

 part in bringing about this alteration. 



10. The investigation of the cytology of our crosses has been undertaken by 



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