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The current status of knowledge and theory pertinent to sex determination and differentiation in teleost fishes was reviewed in advance (Harrington, 1974). The review is critical and selective but based on an inclusive examination of contingent, derivative, and source literature. It provides a cogent introduction to this study, which is a search at the morphogenetic level for the earliest visible manifestations of the differentiation of ovotestes versus primary testes in fish of the same genotype. The present report begins with a retrospective summary of intersexuality in Rivulus marmoratus Poey and of the evidence for the genetic uniformity of our material and its consequences. The new information appropriate to the title of this paper has its context in findings published during the last 14 years by the writer and his co-authors.