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Show 582 DR. W. G. RIDEWOOD ON TnE HYOBRANCHIAL [-MRV 18, carefully freed from the skull. The oesophagus was then cut in front of the liver, and the whole of the floor of the pharynx thus liberated. The heart and larynx were then dissected away, the gills, muscles, and mucous membrane removed, and only sufficient connective tissue left to keep the skeletal parts in their natural relations. No staining reagents were employed, and the whole of the dissection was performed under a simple microscope of a magnification of ten diameters, in powerful reflected light. The series of larvae examined was very extensive, but it will suffice to select eight stages only of these for description. The important changes in the hyobranchial skeleton do not begin until the tail is reduced to about one-third of its maximum length, but they then proceed with considerable rapidity. In fact, two larvae exactly similar in external appearance and having the merest stump of the tail remaining may show considerable differences in the structure of the hyobranchial skeleton. In such cases it is necessary to make the dissections first and to arrange tbe specimens in series afterwards. The youngest tadpole I have been able to examine is one with the hind limbs just appearing as buds on the surface of the body, but from this onwards, as far as the adult condition, the series is as complete as could be wished. It should be noted that the hyobranchial skeleton of the first seven stages is drawn from the dorsal side, in order to show the spicula and the articular ends of the hyoidean cornua. Figs. 8 and 9, howTever, exhibit the ventral surface, because the spicula have by this time disappeared, the hyoidean cornu has no longer an articular surface, but is firmly adherent to the auditory capsule, and because the interesting ventral ossification is only to be seen in this view. In order to facilitate comparison the figures are not drawn to the same scale, but as nearly as possible of the same absolute size. The approximate magnification is given in each case. The introduction of the mandible into the figures senres to show not only the gradually increasing size of the gape, but also illustrates the interesting manner in which the distal extremities of the hyoidean cornua get pushed farther and farther back as the mandibular rami elongate posteriorly. Throughout this contribution, although ostensibly devoted to a consideration of the hyobranchial skeleton, I have purposely avoided all reference to the columella auris and stapedial cartilages. STAGE 1. Distance from snout to root of tall, 13 mm. Length of tall, 16 mm. Length of hind limbs, 1 mm. (Plate X X X V . fig. 1.) There is nothing very remarkable about the hyobranchial skeleton of Pelodytes in its early stages, since it conforms tolerably well with what might be considered the normal for tadpoles in general. There is but a single unpaired median constituent, and not two, as, for instance, in Alytes. Although Parker (12) speaks of this skeletal part of the Anuran larva as the "basibranchial" and Stannius (19. p. 64), Hoffmann (7. p. 45), and Gaupp (5) apply to it the non-committal name "copula," the general |
