Standard tables on the normal development of various laboratory and research animals undoubtedly possess considerable merit not only for embryologists, but also for biologists in other fields. Thus the tables of Oppenheimer ( '37) for teleosts (Fundulus heteroclitus), of Harrison (unpublished) for urodeles (Amblystoma punctatum), of Pollister and Moore ('37) and Shumway ( '40) for phaneroglossan Anurans (Rana sylvatica and R. pipiens, respectively) have already had the widest use. Not only does each set of tables come closer to a quantitative definition of the development of the animal concerned than does verbal description, but such tables also provide the only valid, and at the same time, ready means of comparing the development of various animal forms, and the larger groups they belong to.
It is with this in view that the present set of tables has been drawn up for the anuran Xenopus laevis. The animal belongs to the relatively small but interesting group of tongueless Anurans ( sub-order Aglossa), of which Xenopus, to date, is the only genus known to any extent. An initial study of external embryonic development (including some excellent lithographs of certain stages discussed here) has been made by Bles ('05), while a detailed account of the development and the morphology of the larva was given by Weisz ( '45a, '45b). ; the anatomy of the adult toad had been investigated earlier by Dpeyer ( '13).
Apart from a comparative-embryological viewpoint, Xenopus adults have become firmly established as clinical laboratory animals in several countries, as the most modern pregnancy test forms (Weisman and Coates, '44). Also, the many unique and extremely remarkable embryological and morphological features of the larva, as well as its rapid early development and its transparency (cf. Weisz, '45a, b), promise to make it a highly sought after research animal.