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Show 106 MR. P. E. BEDDARD ON THE [Feb. 6, clefts are slightly oblique. The epithelium which lines it differs on the anterior and posterior faces of the cleft. Anteriorly the epithelium, like that of the buccal cavity, is formed of low cells ; posteriorly it is formed of tall columnar cells. These cells are continuous with the ventral epithelium of the pharynx, which has this character ; the dorsal epithelium being low. This pharyngeal tract of columnar epithelium extends back over the whole of the branchial region, but suddenly stops short a little way in front of the origin of the lungs. This fact is perhaps incidentally of some little importance in view of the homology between gill-slits and lungs which was once urged. Had this modified tract of pharyngeal epithelium extended to the lung and into it, as into the hyoid and branchial clefts, the question might have been considered anew. It will be noted that the hyoid cleft differs much from the branchial clefts which follow in that the modified pharyngeal epithelium only lines its posterior surface. This cleft does not open on to the exterior. In tadpoles of June 2nd (cut longitudinally and horizontally), in which the branchial basket was weU developed with its vascular tufts, the hyoid cleft showed no traces of being a respiratory cleft and did not open on to the exterior either independently or by way of the other branchial cleft. In a tadpole of June 5th, the opening of the hyoid cleft was effected. It has the form of a comparatively narrow tube, which, curving round shortly after its origin from the pharynx, opens into the first branchial cleft a long way from the opening of the latter on to the exterior. Internal Gills.-The branchial arches, as in other Amphibia, fuse to form a basket-work, from the bars of which run cartilaginous processes which become tufted and form the so-called filtering apparatus. I observed the first traces of this filtering apparatus in tadpoles of M a y 31. These structures become later very vascular, and they must be respiratory in function, since no other internal gills are developed. In the C o m m o n Prog the tadpoles possess not only these " filters" but tufted internal gills. Messrs. Marshall and Bles l, while admitting the vascularity of the filters, consider that, " as the blood is returned from them to the somatic veins, it is probable that they are not actively respiratory." They clearly must be in Xenopus, as there are no other gills. External Gills.-As has been already mentioned, the tadpole of Xenopus is said by M r . Leslie to possess no external gills. This statement is not quite accurate, though undoubtedly complex arborescent gills like those of Rana are not to be discovered. Messrs. Marshall and Bles have emphasized the fact, which has been rather slurred over, that the external and internal gills form a continuous series of structures. In 4-5 millim. long tadpoles of Rana " two pairs of external gills are present as backwardly- 1 " The Development of the Blood-vessels in the Frog," Stud. Biol. Lab. Owens Coll. ii. 1890. |