810 MR EDWARD J. BLES ON THE 
toad that the gland is at the height of its development in each of the three species when 
the larva reaches the size at which hatching occurs, and that it rapidly atrophies after 
hatching. Its structure in these forms and in Xenopus is essentially similar. 
The results arrived at on this subject may be summed up by stating that the main 
function of the frontal gland in the Anura is to soften by means of its secretion the 
tough and turgid egg membrane in order to allow the larva to escape at an early stage 
of development before any external hard parts have been formed which might be used 
for breaking out. The frontal gland is a transitory structure, like the ege-tooth of 
lizards, and like it again only actively functional for a few minutes in the life of each 
individual. Its interest is chiefly physiological, but it may serve as a warning to 
morphologists of the danger run in assuming that an inconspicuous organ, the function 
of which is not known, is vestigial. 
The Larve after Hatching.—The first few days after hatching are spent by the 
tadpole attached to weeds, etc. Its abdomen contains a considerable amount of 
yolk which must be absorbed and the alimentary canal opened up before it can begin 
to swim about and feed. This does not come about for a period varying between » 
three days and a week in an aquarium kept at 22° C. The cement organ is 
functional during the whole of this time. The tadpoles at this stage never lie on 
the bottom of the aquarium, where, owing to their pale colour, they would stand out 
conspicuously against the sediment. Those found on the bottom are prematurely 
hatched younger larve. The secretion of the cement gland is extremely tenacious. 
If a tadpole is induced to move, the mucus thread breaks away from the surface 
of the sucker and the animal swims with a curiously stiff flickering wriggle, remarkably 
like that of a young Ampioxus. For the first day or two after hatching a free- 
swimming larva always tends to assume the vertical position with its head up, and 
consequently the swimming movements carry it towards the surface. Another 
physiological character tends in the same direction. Although not clearly discernible 
at any given time in any particular individual, the larvee respond to light stimuli 
in such a way that it could be put down to feeble positive heliotropism. If an 
aquarium containing some hundreds is disturbed and the larvee scattered, the 
great majority of them will, in twenty-four hours, be near the surface or on the 
light side of the vessel; a few dozen, however, will be distributed at random. The 
mucus from the cement organ is both very adhesive and very tough. Whatever 
the cement organ touches it immediately becomes fixed to and the tadpole comes to 
rest. This applies to so unsubstantial an object as the surface film. A 20-inch 
bell-jar, four or five days after spawning has taken place in it, will have one hundred 
to two hundred larvee hanging from the surface film in the position of the larva in fig. 19, 
Plate III. The thread of mucus is of varying length, and may either be as long as or 
rather longer than the larva. The surface film is, of course, drawn down into a slight 
dimple by the weight of the animal. I have never seen any movement in an attached 
larva, except when in the act of swimming away; it usually hangs down perfectly 
