WALL MORPHOGENESIS INCOSCINODISCUS WAILESIIGRAN AND ANGST. I. VALVE MORPHOLOGY AND DEVELOPMENT OF ITS ARCHITECTURE1

Abstract

ABSTRACTThe morphology of the valve ofCoscinodiscus wailesiiand the development of its siliceous architecture, studied in the SEM and TEM, is compared with valve formation inThalassiosira eccentrica(Ehrenberg.) Cleve. Though the areolae‐architecture of these two species differs in such that the cribrum is proximal and the foramen distal inT. eccentrica, and inC. wailesiithe cribrum is distal and the foramen proximal, the formation of their complex loculate system is similar, displacing a centrifugal growth pattern with respect to the valve, and a proximal to distal, sequentially. During base layer formation a hitherto undescribed rib system outlines the prospective areolae. The vertical differentiation is in principle the same as inT. eccentricaand also the cribra are formed centripetally in relation to the areolae in both species. The location of the cribra at the proximal or distal side, therefore, seems to be of minor importance for the sequence of silica deposition. Variation in girdle bands is discussed with respect to cell division. The prophasic nuclear migration from the interphase position to the girdle bands, where mitosis is performed, seems to be necessary for triggering the formation of the unilateral cleavage furrow that later forms a cleavage ring with excentric position. The divided nuclei migrate with the ingrowing cleavage furrow to the center of the newly created protoplasmic surfaces to initiate valve formation.