VIII. On the experimental hybridization of echinoids

The present work was originally undertaken some four years ago, with the object of finding out how readily some of our common sea-urchins could be crossed, and their larvæ reared to a stage when they assumed adult characters. The forms selected for this purpose wereEchinus acutus, E. esculentus , andE. miliaris . One of our number (de Morgan) had already reared the larvæ ofE. esculentus andE. miliaris , as well as the crossesE. acutus ♀ XE. esculentus ♂ andE. miliaris ♀ XE. esculentus ♂ through metamorphosis. Our early experiments showed that if the right conditions were observed, all the possible crosses between these forms could be reared through metamorphosis. This led to the attempt to discover definite characters in the larval development, more specific and fixed than those hitherto employed as a basis of comparison between normal and hybrid plutei. Moreover, of the three species selected for our experiments, two,E. esculentus andE. acutus , are closely related; while the third,E. miliaris , is but distantly allied to the other two, being usually classed in the separate genus ofPsammechinus . Thus, if the hybrids betweenE. miliaris andE. esculentus , andE. miliaris andE. acutus should prove sterile, on account of the specific differences separating these species, those at least betweenE. esculentus andE. acutus would, we hoped, yield fertile hybrids, which could be investigated in a second generation. In the numerous papers which have appeared on Echinoderm hybridization, hitherto only the characters of the early pluteus, up to the assumption of the eight-armed condition, have been investigated. In the past it has been held that the early larval characters are sufficiently definite to give a clear answer to all questions of inheritance. The larval skeleton has been adopted as a main basis of comparison between normal and hybrid larvæ; unfortunately, the skeleton, like many of the early larval features, is subject to irregular variation. Under unfavourable circumstances the skeleton frequently tends to develop extra rods and spinous processes, which give it a spurious resemblance to that of other species. Therefore skeletal characters cannot always be safely attributed to hereditary influence. In our experiments we soon came to the conclusion that if any definite advance were to be made in this subject, it would be best to abandon the skeleton, and all the early features of the pluteus, as an index of parental influence, and to find more definite characters in the later larval life, and that we should have rigorously to adhere in our work to two general principles.