The details of a new fixation procedure using 40 per cent osmium tetroxide in carbon tetrachloride are presented. This fixative is a good general preservative, gives a higher contrast than the ordinary osmium fixatives, and may also preserve structures that are not otherwise readily revealed. Some possible reasons for the increased contrast are discussed.

Micrographs of the sea urchin spermatozoa treated with the new fixative provide more detailed information on the tail structure than has heretofore been obtainable. This information is summarized in the diagrammatic text-figure.

The sperm tail can no longer be regarded as having a bilateral symmetry, and thus, it is possible to assign an index number to each of the nine peripheral filaments.

The nine peripheral filaments have a complex morphology, each one of them seems to be composed of two subunits that have unequal diameters. The slightly larger subunits are all found in the clockwise direction with regard to the other subunit or are all found in the counter-clockwise direction in the sectioned tail. Each of the slightly larger subunits is at intervals provided with two types of projections—referred to as the arms and the spokes—that extend in respective tangential and radial direction. The arms from one filament may be in actual contact with its neighboring filament through a complex bridge-like formation. There is a quantitative difference between the nine filaments with regard to this bridge.

It is assumed that the eleven tail filaments follow straight paths. Some hypotheses on sperm movement are discussed based on this assumption and on the fact that the oscillations of an actively working sperm tail are in one plane. Probably, the nine peripheral filaments have non-equivalent functions in tail movement.

In the centriole the nine peripheral filaments characteristically appear as triplets in a whorl-like arrangement. It is suggested that the inner part of this triplet is a derivation of the arms. A structural abnormality of the tail is described that is characterized by two or three complete sets of tail filaments within one cell membrane.

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