AXOLOTL, the Mexican name given to larvae salamanders of the genus Amblystoma. It required the extraordinary acumen of the great Cuvier at once to recognize, when the first specimens [v.03 p.0069]of the Gyrinus edulis or Axolotl of Mexico were brought to him by Humboldt in the beginning of the 19th century, that these Batrachians were not really related to the Perennibranchiates, such as Siren and Proteus, with which he was well acquainted, but represented the larval form of some air-breathing salamander. Little heed was paid to his opinion by most systematists, and when, more than half a century later, the axolotl was found to breed in its branchiferous condition, the question seemed to be settled once for all against him, and the genus Siredon, as it was called by J. Wagler, was unanimously maintained and placed among the permanent gill-breathers.
It seemed impossible to admit that an animal which lives for years without losing its gills, and is able to propagate in that state, could be anything but a perfect form. And yet subsequent discoveries, which followed in rapid succession, have established that Siredon is but the larval form of the salamander Amblystoma, a genus long known from various parts of North America; and Cuvier's conclusions now read much better than they did half a century after they were published. Before reviewing the history of these discoveries, it is desirable to say a few words of the characters of the axolotl (larval form) and of the Amblystoma (perfect or imago form).
The axolotl has been known to the Mexicans from the remotest times, as an article of food regularly brought from neighbouring lakes to the Mexico market, its flesh being agreeable and wholesome. Francisco Hernandez (1514-1578) has alluded to it as Gyrinus edulis or atolocatl, and as lusus aquarum, piscis ludicrus, or axolotl, which latter name has remained in use, in Mexico and elsewhere, to the present day. But for its large size—it grows to a length of eleven inches—it is a nearly exact image of the British newt larvae. It has the same moderately long, plump body, with a low dorsal crest, the continuation of the membrane bordering the strongly compressed tail; a large thick head with small eyes without lids and with a large pendent upper lip; two pairs of well-developed limbs, with free digits; and above all, as the most characteristic feature, three large appendages on each side of the back of the head, fringed with filaments which, in their fullest development, remind one of black ostrich feathers. These are the external gills, through which the animal breathes the oxygen dissolved in the water. The jaws are provided with small teeth in several rows, and there is an elongate patch of further teeth on each side of the front of the palate (inserted on the vomerine and palatine bones). The colour is blackish, or of a dark olive-grey or brownish grey with round black spots or dots.
The genus Amblystoma was established by J. J. Tschudi in 1838 for various salamanders from North America, which had previously been described as Lacerta or Salamandra, and which, so far as general appearance is concerned, differ little from the European salamanders. The body is smooth and shiny, with vertical grooves on the sides, the tail is but feebly compressed, the eye is moderately large and provided with movable lids, and the upper lip is nearly straight. But the dentition of the palate is very different; the small teeth, which are in a single row, as in the jaws, form a long transverse, continuous or interrupted series behind the inner nares or choanae. The animal leaves the water after completing its metamorphosis, the last stage of which is marked by the loss of the gills. One of the largest and most widely distributed species of this genus, which includes about twenty, is the Amblystoma tigrinum, an inhabitant of both the east and west of the United States and of a considerable part of the cooler parts of Mexico. It varies much in colour, but it may be described as usually brown or blackish, with more or less numerous yellow spots, sometimes arranged in transverse bands. It rarely exceeds a length of nine inches. This is the Amblystoma into which the axolotl has been ascertained to transform. It is generally admitted that the axolotls which were kept alive in Europe and were particularly abundant between 1870 and 1880 are all the descendants of a stock bred in Paris and distributed chiefly by dealers, originally, we believe, by the late P. Carbonnier. Close in-breeding without the infusion of new blood is probably the cause of the decrease in their numbers at the present day, specimens being more difficult to procure and fetching much higher prices than they did formerly, at least in England and in France.
The original axolotls, from the vicinity of Mexico City, it is believed, arrived at the Jardin d'Acclimatation, Paris, late in 1863. They were thirty-four in number, among which was an albino, and had been sent to that institution, together with a few other animals, by order of Marshal Forey, who was appointed commander-in-chief of the French expeditionary force to Mexico after the defeat of General Lorencez at Puebla (May 5th, 1862), and returned to France at the end of 1863, after having handed over the command to Marshal (then General) Bazaine. Six specimens (five males and one female) were given by the Société d'Acclimatation to Professor A. Duméril, the administrator of the reptile collection of the Jardin des Plantes, the living specimens of which were at that time housed in a very miserable structure, situated at a short distance from the comparatively sumptuous building which was erected some years later and opened to the public in 1874. Soon after their arrival at the Jardin d'Acclimatation, some of the axolotls spawned, but the eggs, not having been removed from the aquarium, were devoured by its occupants. At the same time, in the Jardin des Plantes, the single female axolotl also spawned, twice in succession, and a large number of young were successfully reared. This, it then seemed, solved the often-discussed question of the perennibranchiate nature of these Batrachians. But a year later, the second generation having reached sexual maturity, new broods were produced, and out of these some individuals lost their gills and dorsal crest, developed movable eyelids, changed their dentition, and assumed yellow spots,—in fact, took on all the characters of Amblystoma tigrinum. However, these transformed salamanders, of which twenty-nine were obtained from 1865 to 1870, did not breed, although their branchiate brethren continued to do so very freely. It was not until 1876 that the axolotl in its Amblystoma state, offspring of several generations of perennibranchiates, was first observed to spawn, and this again took place in the reptile house of the Jardin des Plantes, as reported by Professor E. Blanchard.
The original six specimens received in 1864 at the Jardin des Plantes, which had been carefully kept apart from their progeny, remained in the branchiate condition, and bred eleven times from 1865 to 1868, and, after a period of two years' rest, again in 1870. According to the report of Aug. Duméril, they and their offspring gave birth to 9000 or 10,000 larvae during that period. So numerous were the axolotls that the Paris Museum was able to distribute to other institutions, as well as to dealers and private individuals, over a thousand examples, which found their way to all parts of Europe, and numberless specimens have been kept in England from 1866 to the present day. The first specimens exhibited in the London Zoological Gardens, in August 1864, were probably part of the original stock received from Mexico by the Société d'Acclimatation but do not appear to have bred.
"White" axolotls, albinos of a pale flesh colour, with beautiful red gills, have also been kept in great numbers in England and on the continent. They are said to be all descendants of one albino male specimen received in the Paris Museum menagerie in 1866, which, paired with normal specimens in 1867 and 1868, produced numerous white offspring, which by selection have been fixed as a permanent race, without, according to L. Vaillant, showing any tendency to reversion. We are not aware of any but two of these albinos having ever turned into the perfect Amblystoma form, as happened in Paris in 1870, the albinism being retained.
Thus we see that in our aquariums most of the axolotls remain in the branchiate condition, transformed individuals being on the whole very exceptional. Now it has been stated that in the lakes near Mexico City, where it was first discovered, the axolotl never transforms into an Amblystoma. This the present writer is inclined to doubt, considering that he has received examples of the normal Amblystoma tigrinum from various parts of Mexico, and that Alfred Dugès has described an Amblystoma from mountains near Mexico City; at the same time he feels very [v.03 p.0070]suspicious of the various statements to that effect which have appeared in so many works, and rather disposed to make light of the ingenious theories launched by biological speculators who have never set foot in Mexico, especially Weismann's picture of the dismal condition of the salt-incrusted surroundings which were supposed to have hemmed in the axolotl—the brackish Lago de Texcoco, the largest of the lakes near Mexico, being evidently in the philosopher's mind.
Thanks to the enthusiasm of H. Gadow during his visit to Mexico in the summer of 1902, we are now better informed on the conditions under which the axolotl lives near Mexico City. First, he ascertained that there are no axolotls at all in the Lago de Texcoco, thus disposing at once of the Weismannian explanation; secondly, he confirmed A. Dugès's statement that there is a second species of Amblystoma, which is normal in its metamorphosis, near Mexico but at a higher altitude, which may explain Velasco's observation that regularly transforming Amblystomas occur near that city; and thirdly, he made a careful examination of the two lakes, Chalco and Xochimilco, where the axolotls occur in abundance and are procured for the market. The following is an abstract of Gadow's very interesting account. "Lakes Chalco and Xochimilco are a paradise, situated about 10 ft. higher than the Texcoco Lake and separated from it by several hills. High mountains slope down to the southern shores, with a belt of fertile pastures, with shrubs and trees and little streams, here and there with rocks and ravines. In fact, there are thousands of inviting opportunities for newts to leave the lake if they wanted to do so. Lake Xochimilco contains powerful springs, but away from them the water appears dark and muddy, full of suspended fresh and decomposing vegetable matter, teeming with fish, larvae of insects, Daphniae, worms and axolotl. These breed in the beginning of February. The native fishermen know all about them; how the eggs are fastened to the water plants, how soon after the little larvae swarm about in thousands, how fast they grow, until by the month of June they are all grown into big, fat creatures ready for the market; later in the summer the axolotls are said to take to the rushes, in the autumn they become scarce, but none have ever been known to leave the water or to metamorphose, nor are any perfect Amblystomas found in the vicinity of the two lakes."
In Gadow's opinion, the reason why there are only perennibranchiate axolotls in these lakes is obvious. The constant abundance of food, stable amount of water, innumerable hiding-places in the mud, under the banks, amongst the reeds and roots of the floating islands which are scattered all over them,—all these points are inducements or attractions so great that the creatures remain in their paradise and consequently retain all those larval features which are not directly connected with sexual maturity. There is nothing whatever to prevent them from leaving these lakes, but there is also nothing to induce them to do so. The same applies occasionally to European larvae, as in the case observed in the Italian Alps by F. de Filippi. Nevertheless, in the axolotl the latent tendency can still be revived, as we have seen above and as is proved by the experiments of Marie von Chauvin. When once sexually ripe the axolotl are apparently incapable of changing, but their ancestral course of evolution is still latent in them, and will, if favoured by circumstances, reappear in following generations.
Bibliography.—G. Cuvier, Mém. Instit. Nation. (1807), p. 149, and in A. Humboldt and A. Bompland, Observ. zool. i. (1811), p. 93; L. Calori, Mem. Acc. Bologna, iii. (1851), p. 269; A. Duméril, Comptes rendus, lx. (1865), p. 765, and N. Arch. Mus. ii. (1866), p. 265; E. Blanchard, Comptes rendus, lxxxii. (1876), p. 716; A. Weismann, Z. wiss. Zool. xxv. (Suppl. 1875), p. 297; M. von Chauvin, Z. wiss. Zool. xxvii. (1876), p. 522; F. de Filippi, Arch. p. la zool. i. (1862), p. 206; G. Hahn, Rev. Quest. Sci. Brussels (2), i. (1892), p. 178; H. Gadow, Nature, lxvii. (1903), p. 330.
(G. A. B.)
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