No. 419.] THE ANATOMY OF AXOLOTLS. 897 
becomes terrestrial and afterward matures. We have come to 
regard the land forms as mature, when as a matter of fact the 
aquatic form may be just as truly mature, since its organization, 
excepting in a few minor points, is identical with that of the 
land forms. There are two kinds of transformation distinguish- 
able in the metamorphosis of an Amblystoma. One of these is 
in the development of the limbs, the alteration of the circula- 
tory system, the development of the lungs, and the matur- 
ing of the organs of reproduction; these may be considered as 
the primary adult characteristics. In addition to these are a 
number of secondary changes, which do not involve radical mor- 
phological processes, but are largely confined to points in the 
external anatomy. These latter are secondary adult character- 
istics. The whole axolotl. question turns on these secondary 
characteristics ; for, as to the primary changes, they take place 
alike in axolotls, siredons, and salamanders, and one is as truly 
adult as the other. But for some reason an interval has arisen 
in axolotls and siredons between the primary and the secondary 
changes, so that instead of all of them progressing par? passu, 
the secondary changes are postponed, and take place either 
much later or not at all In the neighborhood of St. Paul, 
amblystomas are fully adult when only 100 mm. in length, 
perhaps even less. In the Mexican axolotl, when the animal 
reaches this length, the secondary transformations do not take 
place, though the primary ones do, and the animal thus remains 
aquatic, though adult in all other respects. In the siredons we 
find that the condition is intermediate; instead of secondary 
transformation being indefinitely postponed and not taking 
Place at all, it takes place much later. In specimen No. 10 It 
ms taking place after the animal has reached a length of 235 mm. 
instead of having occurred at only 100 mm. In No. 2, of the 
Same size and apparently from the same environment, it has 
not taken place, and no signs of its approach are visible. 
