Spermatozoa from dextrally and sinistrally coiled races of Limnaea peregra were examined by the use of an electron microscope. Helical fibres in the middle and tailpieces of the sperm were found to be dextrally coiled in preparations from individuals of both races. It is concluded that the genes which determine the direction of spiral cleavage in Limnaea do not control coiling in the sperm.

The structural features of the sperm are discussed in the light of limitations imposed by geometrical considerations on the form of the helical fibres.

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