The Endodermal Origin of Middle Ear Cartilages of Rana

Abstract

In recent years a small, hitherto unknown cartilage has been described in the Amphibia. This develops at metamorphosis in the interval between the facial nerve behind and the quadrate cartilage in front. Stadtmüller 1 described a cartilage lateral to the quadrato-mandibular articulation in Bombinator pachypus and termed it “Cart. paraarticularis.” In the same position, in the members of the higher Anura, the Annulus tympanicus develops. In 2 urodeles (Litzelmann 2 on Triton and Stadtmüller 3 on Salamandra) a cartilage develops along the posterior border of the quadrate. Litzelmann shows that the tubo-tympanal rudiment of the first visceral pouch of the Triton embryo occurs in this position, but later becomes cut off from the pharynx and disappears. Litzelmann concluded that the cartilage which develops later in this position is the representative of the hyomandibular cartilage of fishes, long sought in the Amphibia.

The tubo-tympanal rudiment of Rana was shown by Spemann 4 to develop from the embryonic first visceral pouch. In the present study, the development of the tubo-tympanum during the transformation of the larva into the adult has been followed. During larval life a collection of mesenchymal cells appears about the distal end of the tubo-tympanal bud. These cells appear to be derived from the epithelium of the bud itself, inasmuch as cells from the latter appear to migrate out into the mesenchyma. The latter condenses, and at transformation, 2 cartilages differentiate in it. One becomes the Annulus tympanicus, the other the distal part of the Columella auris.

This was established by the following experiments upon a series of Rana sphenocephala embryos. A cut was made through the ectoderm along the hyomandibular groove, at a stage in which the external gills I and II each consisted in 2 simple branches.