104 ME. F. B. BBDDAKD ON THE [Feb. 6, 



After this the tadpoles show a progressive and rapid increase in 

 length. One o£ June 2nd was 10 mUlim., 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 XIU. 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. An 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 larvse 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 Common Frog at the period when the two 

 suckers have become fused. The coexistence of the suckers and 

 the tentacles would seem to entirely disprove any possible 

 homology between the two structures. In the youngest embryo 

 at my 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 Rnna, 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 his 

 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. He 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 been compared to those of a Siliu-oid fish ^. 

 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 



' Perhaps better to the " nasal barbels " of Myxine and Bdcllostoma. 



