The primary side of a valve is the side that is formed from the initial branch of the sternum of raphid diatoms.
Round, F.E., Crawford, R.M. and Mann, D.G. (1990). The Diatoms. Biology and Morphology of the Genera. Cambridge University Press, Cambridge, 747 pp.
Image Credit: Figure adapted from Round et al. (1990).
The series of drawings, from top to bottom, represents the pattern of silica deposition as a valve is formed. First, a narrow rib of silica, the sternum, is deposited. Next, at the poles and central nodule, the sternum is joined after forming the raphe slit. The primary side of the valve is the side based on the first part of the sternum, sometimes termed the primary rib. The secondary side of the valve is the side based on the joined sternum and may include Voigt dicontinuities.
Image Credit: Steve Main
A light micrograph of a species of Neidium from Iowa. The two arrows indicate the Voigt discontinuities, formed by the incomplete joining of the secondary rib during valve formation.