Advantage was taken of the fact that the eggs of Dendraster can readily be fertilised with the sperm of Strongylocentrotus. Such cross-fertilised eggs have the cleavage tempo of their own species, and are unaffected in this respect by the sperm. Eggs were fragmented by cutting them in two with the micro-needle and fertilised, some lots with Dendraster sperm, some with that of Strongylocentrotus. Subsequently nucleated and non-nucleated fragments showed the same time intervals for cleavage in both lots, intervals identical to those of whole eggs. Whence it is concluded that neither sperm nor egg nucleus has any effect on segmentation tempo, but that the reactions of the cytoplasm alone determine it. The suggestion is made that the cleavage reaction depends upon a substance of granular character in the cytoplasm.

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