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Show 104 MR. E. E. BEDDARD ON THE [Feb. 6, After this the tadpoles show a progressive and rapid increase in length. One of June 2nd was 10 millim., of June 5, 13 millim. A tadpole with fully developed hind limbs was 52 millim. long. A tailed frog (August 18) only 44 millim. The mature tadpole represented in the drawing (Plate XIII. fig. 4) is rather longer ; it was not killed. The sucker has been stated to be absent in Xenopus. This is not the case; I found it not only in the youngest stages, but in larvae of 14 millim. in length ; it gradually disappears, however, as the tadpole grows. A n interesting point about the ventral sucker in this Amphibian is that it is a single structure apparently from the very first. It is certainly median and unpaired in the very youngest larvae, which were 5 millim. in length. In larvae of 7 millim. in length the chin sucker is exceedingly obvious, with a raised circular rim of a brown colour. The circular outline of the sucker in Xenopus contrasts with the horseshoe-shaped outline in the young tadpole of the C o m m o n Frog at the period when the two suckers have become fused. The coexistence of the suckers aud the tentacles would seem to entirely disprove any possible homology between the two structures. In the youngest embryo at m y disposal the sucker in transverse section occupied the whole of the ventral surface of the head, extending back to the level of the eyes. It is composed, as in Eana, of closely set elongated cells of a brownish colour. The cells converge upon the surface, so that in transverse sections through the head the cells are seen to be cut transversely and posteriorly, and to be covered by a layer of non-modified epidermis. The surface of the sucker at the centre is quite flat, and it stands out conspicuously beyond the surrounding integument. The cells of the sucker clearly belong to the outer of the two layers of the epiblast, into which they pass without any abrupt demarcation. In later stages the cells of the sucker get less and less unlike those of the surrounding integument. Prof. Parker's failure to find the sucker was due to the fact that bis tadpoles were too old. I imagine that in tadpoles of such an age as those which he figures there would not be the least trace of these structures. It is curious, however, that Leslie makes no mention of them. H e appears to have examined tadpoles of all ages, and in the youngest stages the sucker could hardly be missed if the tadpoles were examined with a hand lens. Tentacles.-As is well known, this frog has a pair of long tentacles, which have beeu compared to those of a Siluroid fish 1. These spring from the angles of the jaw just above the mouth. They get longer as the larva increases in size. More than once I have observed the tentacle of one side to be bifid. The earliest appearance of the tentacles is in the form of a little process of the integument as yet unconnected with the skull. I found the tentacles in this condition in two tadpoles preserved on June 2nd. In younger tadpoles than this I did not succeed in discovering any 1 Perhaps better to the " nasal barbels " of Myxine and Bdcllostoma. |
