Structural Changes in the Anuran Mandibular Arch during Metamorphosis, with reference to Rana Temporaria.

Abstract

The chondrocranium and the mandibular arch have been studied in seven stages. A summary of the changes in the mandibular arch during metamorphosis is given on p. 517 et seq. These changes are discussed on p. 522 et seq. A summary of the homologies which can be established or suggested by a study of the stages is given on p. 531. There are four new suggestions. The temporal region of the skulls of Ascaphus, Lio-pelma, and Aly tes are redescribed on p. 531, and are discussed on p. 538. Ascaphus is shown to possess (in the adult) separate trigeminal and facial foramina divided by a prefacial commissure; a post-palatine commissure; with less certainty, a complete basitrabecular process articulating with a true basal process; and a trigeminal foramen divided possibly by the root of the ascending process. Liopelma is shown to possess the above structures except for the divided trigeminal foramen, with the difference that the root of the basitrabecular process has been lost. Alytes, Bombinator, and Discoglossus each possess a prefacial commissure. In them, and in all the other Anura, the isolated outer end of the basitrabecular process is probably represented as the pseudobasal process (Gaupp’s basal process). The structures described in sections 5, 6, and 7 of this summary have not previously been given these homologies. The change from the Ascaphus to the Ranid type of temporal arrangement is shown to be due to a collecting together of the branches of nerves V and VII into a single pro-otic foramen. Kruijtzer’s theory of the hyoid suspension of the mandibular arch is criticized. An interpretation is given of the probable evolution of the tadpole mandibular apparatus, and of the metamorphic changes required to produce the adult skull.