The epithelia of the olfactory organ of two cyprinodontoid fish species were studied both by transmission and scanning electron microscopy. The relatively flat floor of the organ is covered by sensory and nonsensory epithelia. The latter is distributed in the form of bands or ridges separating distinct areas of sensory epithelium. Differences between the olfactory organs of the two species investigated related only to the topography and quantitative distribution of the epithelia. Their ultrastructural features are very similar. The nonsensory stratified squamous epithelium contains numerous goblet cells and surface cells provided with microridges. A hypothetical function of the microridges is discussed. The sensory epithelium consists mainly of basal, supporting, and two types of sensory cells, i.e., ciliated and microvillous receptor cells. The cilia exhibitapredominant 9+0 microtubule pattern. Both epithelia are covered by a mucus layer in which all surface structures seem to be embedded. The possible nature, origin, and movement mechanisms of the mucus are discussed.

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Summary The development of the olfactory organ of oviparous and viviparous cyprinodonts is described. Differences between both groups referring mainly to the sequence of formation of the nasal openings are discussed. These differences are thought to be related to ontogenetic characteristics not common to both groups (e. g. the formation of an extra‐embryonic pericardium). Résumé Le Développement de l'Appareil olfactif des Cyprinodontes ovipares et vivipares (Teleostei) Le développement de l'appareil olfactif des cyprinodontes ovipares et vivipares est décrit. Les différences entre Ies deux groupes concernant en particulier l'ordre de la formation des narines font l'objet d'une discussion. On suppose que ces différences ont un rapport avec des caractéristiques ontogénétiques qui ne sont pas communes aux deux groupes (p. e. formation du pericard extraembryonaire). Zusammenfassung Die Entwicklung des Geruchsorgans von oviparen and viviparen Zahnkarpfen (Teleostei) Die Entwicklung des Geruchsorgans von oviparen and viviparen Zahnkarpfen wird besdtrieben. Untersdsiede zwisdlen beiden Gruppen, die hauptsächlich die Reihenfolge in der Ausbildung der Riedloffnungen betreffen, werden diskutiert. Es wird angenommen, daβ diese Untersdtiede im Zusammenhang mit ontogenetisdlen Merkmalen stehen, die nidu beiden Gruppen gemeinsam sind (z. B. der Ausbildung eines extraembryonalen Perikards).