Some Unexpected Results of the Heteroplastic Transplantation of Limbs

The embryos of distinct but closely related species of amphibians often differ from one anotber in certain qualities, such as pigmentation, size, time of appearance of organs, etc., which render them useful in the analysis of the factors of growth and development by means of grafting experiments. Two of our native salamanders, Ambylstoma punctatum Linn. and A. tigrinum Green, afford advantages of this kind in connection with the development of the limbs. In the spotted salamander, A. punctatum, the fore legs begin to show externally at a comparatively early stage and but little later than the external gills. By the time the yolk is resorbed, they are fully developed both in form and in function. On the other hand, in the tiger salamander (A. tigrinum) the fore limbs at this period are small nodules still, scarcely visible superficially. It is not until some weeks later that they reach the degree of development attained by the fore limbs of the punctatum larva at the close of the embryonic period. With respect to the development of most of the other organs the embryos of these two species show much greater concurrence, but there are several characteristic differences t-hat require brief notice here. The tigrinum egg is smaller and contains much less yolk, so that this store of food is exhausted sooner than in the punctatum egg. The tigrmnum larva consequently begins to feed when quite small, although, except for the difference in the fore limbs, the embryos of the two species are approximately at the same stage of development at this period. During the first part of larval life the rate of growth is not very different in the two species, but after five or six weeks, when they have reached a length of about 4 cm., the tigrinum larvae begin to forge ahead rapidly and ultimately attain a much larger size than the punctatum. Sexually mature larvae (axolotls) that have come under my notice measure from 25 to 32.5 cm. in length. The two longest adult specimens mentioned by Cope' measured 25 and 27.5 cm., respectively. In the strain used in the present investigation metamorphosis has not been observed to take place in specimens less than 13 cm. in length, and it often, apparently, does not occur at all, while in A. punctatum it takes place when the larvae are about 5 cm. long. This size may be reached under favorable circumstances in the latter species in about ten or twelve weeks, at which time tigrinum larvae of the same age, kept under similar conditions, are about 6 cm. long, VOL. 10, 1924 no