The trichurid egg-shell: evidence in support of the Bouligand hypothesis of helicoidal architecture.

Electron microscopy of thin sections and freeze etch replicas of the eggs of the nematodes Trichuris suis and T. muris is used to provide evidence in support of the Bouligand hypothesis of helicoidal architecture. The evidence presented is as follows: 1. The specific objections to the Bouligand model raised by Dennell (1974) and Dalingwater (1975b) are answered by reference to a pyramid of helicoidal tissue in which the corners are blunt. 2. Sections cut normal to the plane of the laminae do not show parabolic patterning. Parabolae appear if the section is tilted--their direction depending upon the direction of tilting. 3. Freeze etching allows the direct visualization of helicoidal architecture. Fibres are parallel within any one lamina but the fibre direction rotates by an angle of 9 degrees in successive laminae. Parabolic arcs are made up of short lengths of straight fibres--curved fibres were not observed. Planes of sectioning producing single and double spiral artifacts are described and the formation of these artifacts discussed. The sense of rotation of the helicoid is shown to be asymmetrical about any mid-plane through the egg.