Summary. The possible significance of the developmental features of Leiopelma and Ascaphus is best considered in relationship to their systematic position amongst frogs. Nobls'e (1931) recognition of the primitiveness of these two genera has been upheld and confirmed by subsequent investigators. For example:— 1There are 9 presacral vertebrae in Leiopelma and Ascaphus, but not more than 8 in any other living frogs. 2Leiopelma and Ascaphus agree with primitive urodeles in that the interdorsals and interventrals remain cartilaginous throughout life. The vertebrae are thus amphicoelous, but this condition is not found in any other living Anura. 3In the Anura, the reduction of true ribs has reached an extreme. Only Leiopelma, Ascaphus and the Discoglossid frogs retain ribs in the adult, though Pipids have ribs while larvae (Noble, 1931). In the adult Leiopelma, there are at least two pairs of true ribs, and sometimes indications of a third pair are visible. 4Abdominal ribs were well developed in the extinct Branchiosaurs. Such ribs appear in the myosepta of the M. rectus abdominis of Leiopelma, and traces occur in Bombina (Goette). Similar pieces of cartilage have been described in Necturus, but, according to Noble, in no urodele are they as well developed as in Leiopelma. 5Leiopelma and Ascaphus are more primitive than other Anura in that the adult retains two tail-wagging musclea, the pyriformis and caudali-pubo-ischiotibialis, although the tail has disappeared. (Noble, 1931.) 6Posterior cardinal veins are present in the larvae of frogs and urodeles. In the adult Leiopelma, Ascaphus and Bombina, as well as in some adult urodeles, both posterior cardinal veins and the posterior vena cava occur together. The posterior cardinal veins of the adult Leiopelma are very well developed. The above are some of the more important primitive characters of Leiopelma and Ascaphus and in many of them the two frogs show a close relationship to urodeles. Furthermore, many morphological characters previously considered to separate the Urodela from the Anura are now found not to apply to these primitive genera. Their anatomical relationships lend no support to the opinions advanced by Wintrebert (1922), Holmgren (1933, 1939), Save-Soderbergh (1934, 1936) and Herre (1935) and recently upheld by Jarvik (1942) that Urodeles and Anurans have arisen independently of each other. For example, differences in the connections between the quadrate and the neurocranium in Urodela and in Anura seemed to demonstrate a gap which is now bridged by Ascaphus (Pusey, 1938, 1943) and by Leiopelma. The separation of the nasal capsules and the presence of an ethmoidal region of the cranial cavity in Leiopelma provides a condition intermediate between the two orders. The course of the hyoman-dibular nerve in Leiopelma displays relationships more comparable to those pertaining to Urodela than to Anura. But it is not only in many details of its adult anatomy that Leiopelma resembles urodeles; there are also striking resemblances in its development. The similarity is well shown by summarizing the developmental features and comparing them with those of urodeles laying large yolky eggs that undergo lengthy intracapsular development either in water or on land.