Ontogeny of the retina and optic nerve in Xenopus laevis. I. Stages in the early development of the retina.

A staging series for retinal histogenesis is described for Xenopus laevis. It is based on scanning EM studies of intact eyes and serial section analysis of silver-stained preparations. Retinal development is arbitrarily divided into twelve stages (I--XII) based on distinct morphological and histological changes. In general, four different phases of retinal development can be identified: an embryonic phase of rapid histogenesis (Nieuwkoop-Faber stages 21--39) and onset of visual function; a second or functional phase completed by stage 52 is a period of growth, synaptogenesis, and the organization of visual function; a third phase ending at metamorphic climax (NF57) when mature function is achieved and binocular vision begins; and a final mature phase at the end of metamorphosis, when road and cones are adult size and the visuotectal projection extends over the entire tectum. Ten of the twelve developmental stages are assigned to the first phase since this is the period of intense morphogenesis and differentiation.