The tadpole ceases to feed, the whole intestinal canal is voided of its contents, and by "histolysis" is thoroughly rebuilt, becoming wider and shrinking to about one-sixth of its original length,–undoing thereby the spiral–preparatory for the coarser food, which consists of insects, worms, and other strictly animal, living matter. Hitherto the tadpoles have lived on "mud," confervae, Diatoms, rotting vegetable and animal matter. The anal tube collapses, becomes ultimately absorbed, and a new vent is formed at and below the root of the tail.
Barfurth[[33]] has made interesting observations and experiments with regard to the absorption of the tail and other organs which disappear during the metamorphosis. This is retarded by low temperature; it is accelerated by rest and freedom from mechanical disturbances, as, for instance, concussion of the water. Hunger shortens or hurries on the last stages of metamorphosis, the absorption of the tail taking place in four instead of five days. Amputation of the tail has no retarding influence; it is followed at once by regeneration, although the tadpole may be on the verge of reducing the tail. Whilst hungering the whole organism draws upon its available store of material, naturally first upon those parts which sooner or later are to become superfluous. This applies eminently to the tail, which represents a considerable amount of "edible" matter, and also to that portion of the skin which still covers the fore-limbs. The elements of the cutis are resorbed, thereby thinning the skin; and consequently the limbs break through earlier in fasting than in well-fed specimens. Nature herself seems to apply hunger as an accelerator. Mlle. von Chauvin found that the larvae of Urodela normally fast during the transformation, and according to Barfurth the larvae of Rana temporaria eat less after their hind-limbs are fully developed. This is, however, also preparatory for the reorganisation of the gut, which has to be more or less empty during the shortening process.
The loss of the tail is not due to a sudden dropping off of this organ–a crude but by no means uncommon belief–but is brought about by a very gradual process of resorbtion. When the fore-limbs begin to break through the skin, the tip of the tail shrinks and becomes black, owing to an increase, or rather concentration, of the pigment cells. The reduction proceeds from the tip forwards until on about the fifth day there remains only a short, conical, black stump. From the beginning of this process of reduction the tail is scarcely used for locomotion, the tadpole rowing with its legs, or it crawls and hops about, although the tail may still be 20 mm. long. The cells of the epidermis atrophy, shrink, and peel off, while those of the cutis, blood-vessels, nerves, muscles, and chorda dorsalis become disintegrated, often undergoing fatty degeneration. The leucocytes eat up the débris and other dissolved tissue, and carry it away through the lymphatic vessels, to be used as new building material in the rest of the animal.
Barfurth asks very properly, Why do these tissues degenerate and die? Because the vasomotor nerve-fibres cease to regulate the circulation. And why does this trophic influence of the central nervous system stop? Because the function of the tail becomes superfluous through the appearance of the fore-limbs. The tail is doomed, and degenerates like any other organ without a function. The whole process is, of course, a recapitulation of ancestral, phylogenetic evolution.
CHAPTER III
NEOTENY–REGENERATION–TEMPERATURE–GEOGRAPHICAL DISTRIBUTION
Neoteny.–It has long been known that the larvae of the Spotted Salamander occasionally attain the size of 80 mm. or about 3 inches, whilst the majority undergo metamorphosis when they are only 40 mm. long. Again, larvae of Triton have been found, in the months of April and May, 80 to 90 mm. long, still with functional gills, but with the sexual organs fully developed. De Filippi[[34]] found in one locality in Lombardy, besides a few normal fully metamorphosed specimens of only 30 mm. in length, more than forty specimens, which, although they had attained full size, about 55 mm., and were sexually mature, still retained their gills. According to him such gill-breathing, otherwise mature specimens, occur constantly in a small lake in the Val Formazzo, on the Italian slope of the Alps, in the province of Ossola. Later Duméril[[35]] astonished the world by his account of the metamorphosis of the Mexican gill-breathing Axolotl into an entirely lung-breathing and terrestrial creature, hitherto called Amblystoma, and supposed to be not only a different species, but to belong to a different family from the Axolotl, which was known as Siredon axolotl s. pisciforme, and naturally classed with the Perennibranchiata.
This discovery led to a series of observations and experiments, chiefly conducted by Marie von Chauvin, instigated thereto by Koelliker and by Camerano.[[36]] It was then found that many, if not most of the European Amphibia, both Urodela and Anura, occasionally postpone their metamorphosis, and also that such Urodela sometimes become adult for all practical purposes, but retain their gills.
This retardation, the retention of larval characters beyond the normal period, was called Neotenie by Kollmann[[37]] (νέος, young; τείνω, extend, stretch). He distinguished further between:–I. Partial Neoteny, namely, simple retardation of the metamorphosis beyond the normal period, for instance, the wintering of tadpoles of Pelobates fuscus, Bombinator pachypus, Pelodytes punctatus, Alytes obstetricans, Hyla arborea, Rana esculenta, R. temporaria, Bufo vulgaris, and B. viridis: II. Total Neoteny, where the animal retains its gills, but becomes sexually mature; hitherto observed in Urodela only, e.g. Triton vulgaris, T. alpestris, T. cristatus, T. boscai, T. waltli and Amblystoma. Intermediate stages between these two categories are not uncommon.
A satisfactory explanation of the meaning of neoteny is beset with difficulties. Some authorities look upon the phenomenon simply as the result of adaptation to the surroundings, which make it advantageous for the creature to retain its larval features. Others think that the surroundings somehow or other retard or prevent the assumption of the adult characters. Undoubtedly there are many cases in which larvae have been reared in water-holes with steep walls, so that they could not change from aquatic to terrestrial life, and it stands to reason that abnormally forced and prolonged use of the gills and of the tail may stimulate these organs into further growth at the expense of the limbs and other organs which are intended for terrestrial life. But not unfrequently typical neotenic and overgrown specimens occur side by side with others which have completed their metamorphosis, and the same is true of larvae of newts which were reared, for experimental purposes, under exactly the same conditions–for instance, in a high-walled glass vessel.