816 MR EDWARD J. BLES ON THE 
D the right and left tentacles are symmetrical as far as the general arrangement of the 
branches is concerned, but the position and sizes of the branches differ on the two 
sides, so that the symmetry is very imperfect. In C the right tentacle has a secondary 
branch on its backwardly directed fork which is not represented on the left side. 
Branched tentacles have only been found in the late tadpole stages over 50 mm. long, 
As BouLrencEr pointed out (footnote to LEsiiz, 90), the tentacle of Xenopus may 
be homologised with the balancers of Urodele larvee. These must be the representa- 
tives on the mandibular arch of the external gills on the branchial arches. If these 
homologies are correct, the tentacle in Xenopus is an external gill, and this conclusion 
is supported by the fact that from its very earliest appearance it has a capillary loop 
doubling into it, supplied from the dorsal end of the first branchial aortic arch. The 
branches figured above are, on this hypothesis, the result of a tendency to branch ma 
persisting gill similar to that found in all external gills which persist for some time 
during the life or, as in Proteus and Siren, for the whole of the life of the individual. 
In the last two animals the tendency to form branches and secondary branches is 
especially well marked. If the branching is put down to regeneration after injury, how 
is the bilateral symmetry to be accounted for ? 
Coloration.—A most remarkable feature in the behaviour of the chromatophores is 
found in tadpoles of 15-18 mm. and onwards. As is not uncommon among tadpoles, the 
dark chromatophores on the head and trunk contract at night into spherical masses, but 
what is most unusual, if not at present unparalleled in any vertebrate, is the fact that 
other chromatophores, apparently of the same nature, namely, those in the distal half of 
the tail, expand at dusk as the others are contracting. 
The end of the tail, which in the day-time is so transparent that the presence of 
chromatophores would never be suspected, becomes, to the naked eye, jet black after 
nightfall. The expansion takes place in the chromatophores in the fin-fold, but not in 
those on the myotomes of the same (distal) part of the tail; the latter contract at night 
in harmony with those in the trunk region and anterior myotomes of the tail. The 
general effect is well shown in fig. 24, although the contrast is much stronger in the 
living animal, where the pale regions become of glassy transparency. The explanation 
is, I believe, due to a need for protective colouring in the transparent part of the tail 
tip. Itis kept undulating constantly ; in the daylight it needs no pigmentation —it is 
protected by its transparency ; but the refractive stellate cells of the mesenchyme in the 
fin-fold would at night be lable to catch and reflect any stray light rays, and the ex- 
panded chromatophores effectively prevent this. Moreover, they are absent in the part 
of the fin-fold which does not move actively. But when the physiology of the case is 
considered it seems to make the solution of the problem of control over the pigment cells 
more dificult than ever. Taking tadpoles from daylight into a darkened room has the 
same effect as the changes from day to night. 
In figs. 20 and 21, Plate III, the appearance of the skin is shown under the microscope 
of the part of the tail marked with a cross in fig. 24, in a specimen killed in the daytime 
