Abstract
The following are the chief conclusions arrived at in this paper: The hybrid larvae obtained on crossingSphaerechinus ova withStrongylocentrotus spermatozoa vary in type at different seasons of the year. Thus in the summer months, when the sexual products ofStrongylocentrotus are at their minimum of maturity, the hybrids show a greater resemblance to pureSphaerechinus plutei than do those obtained in the spring. Many more of the larvae have cross bars between their anal arm skeletons, and the arms themselves are longer whilst the bodies are shorter. However, practically none of the summer hybrids were of a pure or nearly pureSphaerechinus type, as they had been found to be on a previous occasion. Contrary to the conclusion of O. and R.Hertwig, it was not found that keeping the ova ofSphaerechinus in sea water for some hours before crossing them withStrongylocentrotus sperm, increased the amount of cross fertilisation. On the contrary, the number of plutei obtained under such conditions was distinctly less than when both sexual products were fresh. In the reciprocal cross ofStrongylocentrotus ♀ withSphaerechinus ♂ the amount of cross fertilisation was found to be considerably increased by this means. Hybrid larvae betweenEchinus acutus ♀ andStrongylocentrotus and alsoEchinus microtuberculatus ♂ are described.
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