Longitudinal fragments of the hypotrichous ciliate Stylonychia were studied a t various stages of cortical regeneration by Scanning Electron Microscopy. Analysis showed that (except for the oral primordium) all primordial fields responsible for replacing excised ciliature develop in continuity with existing primordia; i.e., all ciliary primordia are derived from segments originally continuous with the oral primordium. This observation leads to the concept of a single primordial field, a "primordium of the ciliature," possessing the developmental competence of giving rise to a complete set of ciliary structures. Such a concept implies the operation of a reciprocal intra-cortical communication system; i.e., the developmental manifestations of any individual primordium are determined a t least in part by the precise location of morphostatic and/or primordial ciliary structures. An intra-cortical communication system is consistent with the previously proposed "determinative region" (Grimes, '75), the function of which is to serve as the initiation site of a primordium during excystment morp hogenesis.Morphogenesis and control of cortical patterning in ciliates have been subjects of several extensive studies and reviews (e.g., Frankel, '73, '74, '75; Grimes, '72, '73a,b,c, '76; Hammersmith, '76a,b; Hashimoto, '61, '62, '63, '64; '63, '72a,b, '74; '72, '75; '69; Sonneborn, '63, '70, '75; Walker and Grim, '73). In a n attempt to understand the factors which control morphogenesis in hypotrichous ciliates, various studies, most notably those of Hashimoto ('61, '64) and '69, '741, have approached the problem through observation and interpretation of morphogenetic patterns during cortical regeneration (the morphogenetic sequence initiated by cell trauma resulting in the replacement of all missing cortical structures). To date, these studies have employed light microscopy to examine the results of cortical excision and subsequent regeneration.In an attempt to determine the morphogenetic interactions among existing cortical structures and primordia in Stylonychia, longitudinal cell fragments were examined a t various stages of regeneration by Scanning Electron Microscopy (SEMI. The SEM permits a 3-dimensional analysis of external cortical events during morphogenesis without the problems associated with protargol staining (irreproducibility leading to a limited number of well-stained cells). This SEM analysis demonstrates the ability of the oral primordium to participate in the formation of primordial fields which, in the undisturbed cell, would utilize other components of the existing ciliature. These observations and their relation to current theories on morphogenetic mechanisms are discussed. (Grimes, ' 7 3~) .
MATERIALS AND METHODSThe anterior portion of one half of individual doublet cells was removed microsurgically, thus inducing cortical morphogenesis in both halves of the doublet (regeneration on the damaged half, morphogenesis comparable to reorganization on the intact half). Cortical reorganization o...
