The problem as to whether Strongylocentrotus pallidus (G. O. SARS, 1871) is a distinct species or only a colour-variety of S. droebachiensis was subjected to a renewed investigation. For the first time simultaneous experiments on homologous and heterologous fertilization of S. droebachiensis and S. pallidus were carried out. These experiments revealed the existence of clear specificity barriers between the two forms. Studies of mitosis in hybrid embryos indicated that chromosomal aberrations are frequent. Electron microscopic investigations demonstrated that the ultrastructure of the cortical granules is different in the two species.The differences observed supply strong evidence in support of the opinion that Strongylocentrotus pallidus is a good species.

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