A. jeffersonianum.–This very slender and slippery species, reaching a length of 6 inches, is remarkable for its long fingers and toes, and its rather compressed tail. The general colour is brown above, dirty whitish below, generally with numerous, small, light blue and pale brown spots on the sides of the neck, body, limbs, and tail. There are several colour-varieties, one of them with white specks. It is a very active and surprisingly good climber, easily escaping out of high-walled bell-glasses, hiding in the daytime in dark and moist localities. Its range extends from Indiana and Virginia to Quebec.
A. persimile.–This species is remarkable on account of its geographical distribution. It is the only non-American species, inhabiting the higher mountains of Siam and Upper Burmah. There is no doubt about its belonging to the genus Amblystoma, although it had originally been described as a Plethodon. It closely resembles A. jeffersonianum in most of its characters, notably in the arrangement of the palatal teeth, general proportions, slender toes, and even in the presence of whitish spots, which are scattered over the sides of its blackish, smooth skin.
A. tigrinum.–This, the commonest species, is conspicuous for its large, depressed head, which is as broad as it is long, its width being enhanced by the unusually large parotoid glands. The mouth is very wide. The large, prominent eyes are golden, and reticulated with brown. The gular fold is strong. The limbs are stout, the fingers and toes short. The trunk is strongly constricted by twelve intercostal grooves. The tail, which is as long as the rest of the body, is somewhat compressed laterally, but bears no trace of a fin. The general colour is more or less dark brown or bluish black, marked with numerous yellow spots and large blotches; the under surface inclines to grey. The length of the adult male is about half a foot; the females, as usual being larger, sometimes reach the length of 9 inches. The range is from New York to California and to Central Mexico.
The larva of this species is the famous Axolotl. It is provided with three pairs of delicate and much-branched external gills, a flat, long tail with a broad ventral and dorsal fin, the latter extending along the back almost to the neck. The limbs, although comparatively slender, are fully developed, and the head is much more pointed than it is in the perfect form. The larvae usually reach 8 or 9 inches in length; exceptional specimens have been recorded of one foot in length, and have been described as Triton ingens.
Fig. 22.–Axolotls or larvae of Amblystoma tigrinum. × ½.
These larvae were found by the Spanish conquerors to occur in great numbers in the lakes near Mexico City, and were called Axolotl by the natives, a word signifying "play in the water." They were, and are still, eaten, either roasted or boiled, with vinegar or cayenne pepper.
For many years these creatures were looked upon as a species of the Perennibranchiata, under the generic name of Siredon (S. axolotl, s. pisciformis, s. mexicanus, etc.), although Cuvier suspected that they were but the larvae of an otherwise unknown terrestrial Urodele. The mystery was not cleared up until the year 1865, when some Axolotls which had been kept for a year in the Jardin des Plantes at Paris, suddenly began to pair, and laid eggs which within six months developed into full-sized Axolotls. This certainly looked as if these creatures were not larvae, but a true Perennibranchiate species. But to the general surprise several of these young Axolotls gradually lost their gills, the clefts closed up, the fins of the back and tail disappeared, the head became broader, the creatures left the water permanently, and in fact turned into the already well-known terrestrial Amblystoma tigrinum. The other brothers and sisters of the same brood remained aquatic Axolotls, which thereby revealed themselves after all as the larval and not as the perfect stage of this remarkable species.
At the suggestion of Kölliker and Weismann, Frl. Marie von Chauvin[[54]] undertook, at the University of Freiburg, long and carefully conducted experiments, showing (1) that little Axolotls can comparatively easily be caused to develop further into the perfect Amblystoma if they are induced to breathe air more frequently than usual; shallow vessels, perhaps also insufficiently aerated water, will produce the desired result; (2) that the commencing metamorphosis can again be checked, the shrinking gills then undergoing fresh development; (3) that they can be forced to remain Axolotls; (4) that the cutting off of the gills has no influence upon their possible metamorphosis, the gills being easily and quickly renewed. The same lady found also that Amblystoma, the perfect form, lives in the water during the pairing time and behaves in the same way as the Axolotls.
The latest observations have been made by Metzdorff.[[55]] Axolotls, at least those which are kept in captivity in Europe, are ready for propagation several times in the year, either in the spring, from April to June, or in December. The male deposits spermatophores, which in the following night are taken up by the female into the cloaca. On the following day, preferably in the afternoon, she grasps a suitable leaf, for instance that of Vallisneria, with the hind-limbs, and presses it against the vent. The eggs are expelled by strong wriggling movements of the body, and are formed into three or four packets of six to ten eggs each, so that about thirty eggs are laid at one sitting. Then she takes a rest before proceeding again; the whole process, in which the male takes no further interest, lasting about two days. The most suitable temperature is one of 18-20° C., or about 68° F. The water must be well aerated. Sterile eggs turn white on the second day. The little larvae are hatched in about a fortnight. Eggs which are kept in a higher temperature, from 22-24° C., develop more quickly, but the resulting young are smaller; they show already on the fifth day head, tail, and the beginning of the gills. According to Bedriaga, they live at first upon Infusoria and Daphnia; when they are 20-25 mm. long they eat Tubifex rivulorum; later on they take scraped meat and are liable, when hungry, to nibble off each other's gills, but these are easily reproduced. When 20-25 cm. long, at the age of about six months, they are able to breed. The chief point of interest is the fact that this species of Amblystoma frequently remains throughout life in the larval state, except that it develops generative organs. The natural causes of this retention are not completely known. According to Shufeldt, who observed them under natural conditions near Fort Wingate in New Mexico, plenty of food, the drying up of the swamps, and the increasing temperature of the diminishing water, hurries on the metamorphosis, while deeper water retards it. Weismann[[56]] suggested that the specimens in the Mexican lakes which remained Axolotls were prevented from becoming perfect Amblystomas on account of these lakes, after the disappearance of the surrounding forests, having receded from their former boundaries, which are now covered with a saline, uninhabitable crust. This may be an explanation, although Axolotls do not live in brackish water. But Weismann went farther, and with his well-known dialectic powers has succeeded in spreading the belief not only that the Axolotl is a case of reversion to an ancestral stage, but that the present Amblystoma, instead of being the progressive, perfect form, is likewise a case of reversion. A reversion from a reversion! The whole line of evolution would then be as follows: Amblystoma; its young, owing to adverse circumstances, revert to the stage of the Perennibranchiate ancestors of all Urodela; if some of these Axolotls lose their gills and fins, they revert thereby into the original Amblystoma. Surely a roundabout way of explaining a curious but after all rather simple process of Neoteny; cf. p. [63].