A role for cytoplasmic determinants in the early development of several organisms has been suggested by results from a variety of studies. Ideally, a demonstration of the existence of determinants requires their transfer between regions of an embryo, but this has been achieved only rarely (see chapters by Nusslein-Volhard and Jáckle, this volume). In many cases, the existence of cytoplasmic determinants is inferred indirectly, for example where procedures that disturb the asymmetric arrangement of the cytoplasm also disrupt the normal allocation of cell types, or where a correspondence exists between the prospective fate of a cell and the cytoplasmic endowment it has received (see chapters by Gurdon, Schierenberg, Strome and Kalthoff, this volume). In these latter examples, a heterogeneity within the cell may indeed exist and influence further development, but its nature may preclude direct testing by cytoplasmic transfer, or a regulatory capacity may exist whereby the localization and/or expression of putative determinants can be modified (Jeffery & Raff, 1983; Johnson & Pratt, 1983).

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