We describe a series of stages for development of the embryo of the zebrafish, Danio (Brachydanio) rerio. We define seven broad periods of embryogenesis--the zygote, cleavage, blastula, gastrula, segmentation, pharyngula, and hatching periods. These divisions highlight the changing spectrum of major developmental processes that occur during the first 3 days after fertilization, and we review some of what is known about morphogenesis and other significant events that occur during each of the periods. Stages subdivide the periods. Stages are named, not numbered as in most other series, providing for flexibility and continued evolution of the staging series as we learn more about development in this species. The stages, and their names, are based on morphological features, generally readily identified by examination of the live embryo with the dissecting stereomicroscope. The descriptions also fully utilize the optical transparancy of the live embryo, which provides for visibility of even very deep structures when the embryo is examined with the compound microscope and Nomarski interference contrast illumination. Photomicrographs and composite camera lucida line drawings characterize the stages pictorially. Other figures chart the development of distinctive characters used as staging aid signposts.

By observing numerous living eggs from the lesser spotted dogfish Scyliorhinus canicula (L.) caught near Roscoff (France) and reared at recorded temperatures as they developed from first cleavage to hatching, the first reasonably complete developmental table was worked out for this classical material in vertebrate embryology. The successive stages, described and numbered from 1 to 34, correct and replace the incomplete stages A-Q proposed by Balfour (J. Anat. Physiol., 10:555-576,1876) and other even less complete series later published, and is unique in the inclusion of a timetable at 16°C. The stages can be identified, usually through the cleared eggshell wall, with naked eye or low magnification. Normal-series stages have been published for most of the animal species widely used in experimental embryology? as an aid to others in confirmation of results. An attempt is usually made to mark each stage by the first appearance of some easily recognized structure or event; the usefulness of a series depends on the closeness and obviousness of the steps, and upon a timetable of their appearance under defined conditions such as temperature. For example, landmark series in continued use are those for Ambystoma (essentially finished by Harrison in 1927 and widely distributed privately before publication in 19691, for the chick (Hamburger and Hamilton, ,511, Xenopus (Nieuwkoop and Faber, ,561, Pleurodeles (Gallien and Durocher, '57), the sturgeon (Dettlaff and Ginsburg, '54), and Salmo (Vernier, '69). Others have been published as necessary adjuncts to particular researches on less commonly used species with narrower focus, e.g., Ballard ('86) on A m i a calua, following earlier studies on several teleost species. Such normal development series for a large number of both invertebrate and vertebrate species has been brought together by Dettlaff and Vassetzky (originally in Russian, later translated to English, '90) but still not including any elasmobranch species.Earlier such collections usually included wider gaps and lacked timetables, though serving well the consolidation of comparative descriptive embry-ology. In the first of several volumes edited by Oskar Hertwig and intended to summarize vertebrate embryology, Franz Keibel('O6) re-issued the illustrations of already published normal series of representatives of all classes from cyclostomes to mammals, whose descriptive embryology was being synthesized by the other authors.At that time the single major study of elasmobranch development was that of Balfour (1876). Despite the high reputation of that author, the defects of his normal series were well recognized. Earliest and latest stages of development were inadequately covered, his Stage E was admittedly abnormal, and he had had to fill gaps in what was otherwise a series for "Scyllium" (now Scyliorhinus) with specimens of the shark "Pristiurus" (now Galeus melastomus) and the electric ray Torpedo. No time sequence could be offered for any of his haphazard collections.In spite of this, the Balfour Stages A-Q...