This paper deals with processes occurring in the assembly of genic products into cell organelles and structures, especially with processes determining the initiation, location, orientation and number of cortical structures in the ciliated protozoan, Paramecium aurelia . The examples analysed experimentally are: ciliary basal bodies and their associated structures, the repeating unit territory of cortical structure, the pattern of path directions of the rows of basal bodies and unit territories, the distribution in the cortex of two different kinds of unit territories, and the ingestatory apparatus (vestibule and gullet). Among the processes that appear to occur are homonucleation, heteronucleation, and allosteric transitions, as found by others in the in vitro assembly of bacterial flagella, bacterial viruses, and the parts of ciliary microtubules and their appendages. In Paramecium , new basal bodies can arise only in one position and orientation, close and at right angles to an existing basal body at a specified spot. Unit territories reproduce by forming additional parts and subdividing. The information for the positioning and orientation of the developing parts of the unit territory is located within the unit itself and, when experimentally altered, reproduces in the altered orientation which cannot be corrected by genic action. This hereditary aspect of development is determined by an unbroken chain of self-reproducing arrangements of cortical parts. Search for DNA in the cortex gave negative results. Analysis of the hereditary determination of initiation, location, and orientation of the gullet gave results similar to those on the basal body and unit territory. Analysis of the hereditary determination of the path of the rows of unit territories and basal bodies and of the distribution of the two kinds of unit territories showed developmental and genetic control by spatial relations to the vestibule-gullet. Thus, for all corticaltra its examined, development is hereditarily determined by existing and self-reproducing cortical arrangements: the genes (or DNA) doubtless control synthesis of the molecular building blocks, but not their site of assembly or the position, orientation and number of assemblies. The flatworm Stenostomum also shows hereditary developmental control by self-perpetuating structural arrangements, two variations from normal (two kinds of doublet worms) reproducing true to type during asexual reproduction. Whether similar processes occur in the inheritance of the developmentally decisive organization of the amphibian egg is still an open question The processes described in this paper constitute but one of many options available for hereditary control of development.