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Show 1894.] TADPOLE OP XENOPUS L.EVIS. 105 trace of the tentacles. The tentacles in these tadpoles are in the form of a small process of the body connected with it by a narrower stalk ; it is covered wdth a layer of columnar epidermis, and the interior is filled with a mass of dense tissue. It shows no resemblance to the sucker in its minute structure. A narrow rod of cartilage runs towards it from the ethmoid just above the joint where Meckel's cartilage articulates, but does not reach it. A slip of muscle is attached to the base of the rudimentary tentacle. In a full-grown or nearly full-grown tadpole such as that displayed in the accompanying coloured drawing (Plate XIII. fig. 4) the tentacles are of considerable length, with a slender bar of cartilage running right along them as is figured by Parker (loc. cit. pi. lvii. figs. 1, 2, & c ) . They are inserted so exactly at the angle of the mouth that they are deeply grooved by it. During life a blood-stream can be observed to pass along the tentacles. The histological structure is not in any way remarkable. Beneath the epidermis is a certain amount of pigment. The interior of the tentacle is taken up by a network of connective tissue. On that side furthest away from the body are two blood-channels lying side by side; the axis of cartilage is small relatively to the diameter of the tentacle. Mr. Boulenger, in a footnote appended to M r . Leslie's paper quoted above, compares the tentacles to the " balancers " of Triton and Amblystoma. This can hardly be, if the latter are, as M r . Orr states l, the homologues of the external gills belonging to the mandibular arch. Mouth-cavity and Pharynx.-In the newly-hatched tadpole (May 29) the mouth is only a depression not communicating with the gut; there are no gill-slits and no skull. O n the following day the mouth was established. The most important fact with regard to the mouth-cavity has already been established by Parker and Leslie ; that is, of course, the entire absence of the horny larval teeth. To confirm the absence of these characteristic structures by microscopical sections is not, perhaps, an altogether unnecessary piece of work. At no stage in the development of the tadpole of this frog did I succeed in discovering the least trace of the structures in question. In tadpoles of M a y 31 some of the characteristic features of the mouth-cavity and pharynx are already obvious. Just behind Meckel's cartilage is a deep recess of the mouth-cavity ventral in position ; laterally this becomes a narrow slit, close to the cartilage, and appears to be the first visceral cleft, though I have not found any connection with, the exterior. It differs from the succeeding visceral clefts in being directed more forwards, their inclination being at right angles with the longitudinal axis or oblique in the opposite direction. The first branchial cleft lying behind the hyoid arch is deep and narrow. It is at right angles to the longitudinal axis, whereas the succeeding 1 " Notes on the Development of Amphibians, &c," Q. J. M. S. 1889, |