Figure 13. Schematic diagram depicting the initiation and early growth of bosses within the periostracal groove.

Bosses inititate at subcellular positions which are equally spaced comarginally along the outer mantle fold. Growth proceeds as the periostracum slides between the mantle folds by secretions of the outer mantle fold. Below each boss there is a reticulum formed by extensions of the underlying microvilli, which also migrates across the cells in coincidence with the boss. The time of initiation alternates between rows, with the formation of bosses labelled 1 and 2 in the left diagram and of boss 3 in the right diagram. B, boss; Emv, reticulate extensions of the microvilli; MMF, middle mantle fold; Mv, microvilli; OMF, outer mantle fold; Po, periostracum.