Primary side

The side of the valve formed from the initial branching of the raphe sternum, during raphid diatom ontogeny. Also termed primary rib.

See also secondary side.

Valve Morphogenesis
Image Credit: Figure adapted from Round et al. (1990).
This series of drawings represents the pattern of silica deposition as a raphid valve forms. At top, a narrow rib of silica, the primary side of the sternum, forms first. Next, at the poles and central nodule the primary side extends to form the secondary side of the sternum, creating the raphe slit. Finally, both sides complete the valve formation. This secondary side includes a Voigt discontinuity.
Neidium Sp2  L 9 59
Image Credit: Steve Main
Light micrograph of a species of Neidium from Iowa. The two arrows indicate the Voigt discontinuities, formed by the incomplete joining of the secondary side during valve formation.