AbstractIn embryos of the gastropod,Ilyanassa obsoleta, ribosomal RNA (rRNA) synthesis first becomes detectable using sucrose gradients at the end of cleavage (by 24 hours post trefoil at 19°C; the trefoil stage is characterized by maximal extension of the third polar lobe, in association with first cleavage). The nuclei of early embryos, however, contain several prominent nucleolus‐like bodies (NLBs) from the 2‐cell stage onward. Investigation of these NLBs ultra‐structurally and cytochemically has led to a model for nucleologenesis and the regulation of expression of the rRNA cistrons during cleavage. Throughout the cleavage period, NLBs consist of homogeneous, spherical accumulations of 5–8 nm diameter fibrils. By 18 hours post trefoil, they have become extensively associated with chromatin. By 22 hours post trefoil there are definitive, chromatin associated nucleoli which have a distinct, 15–25 nm granular component surrounding and intermingling with a matrix of 5–8 nm fibrils. This period marks the beginning of a reduction in the number of NLBs per nucleus, until most post‐gastrula nuclei contain only one or two definitive nucleoli. Both NLBs and nucleoli stained selectively with bismuth, which Locke and Huie ('77) consider to be specific for basic proteins within the nucleolus. In contrast, a silver staining technique (Bloom and Goodpasture, '76) selective for active nucleolus organizers stained nucleoli of post‐gastrula embryos but not NLBs of mid‐cleavage embryos. Electron microscope autoradiography showed the incorporation of3H‐uridine into NLBs beginning in late cleavage, just prior to the appearance of a granular nucleolar component. The agranular NLBs present in cleavage nuclei appear, therefore, to be precursors of the fibrillar matrix of the nucleoli which eventually replace them. This transition is coincident with the activation of rRNA gene expression as NLBs associate with chromatin towards