Changes in the Life-cycle 315 
Influence of the Environment on the Time of Ripening of the 
Sexual Organs 
In most animals the sexual organs ripen their products when 
the adult form is reached, but in a few cases it has been shown 
that sexual maturity may obtain while the form of the body 
remains in the young or larval state. This phenomenon in 
its various forms has been called neotenia. The classical case 
is that of the axolotl. This salamander becomes sexually ma- 
ture while still leading an aquatic life. It is a large, newt-like 
amphibian, eight to nine or even twelve inches in length. It 
has three pairs of branched external gills, a long tail with a dor- 
sal and ventral fin. The animals were first found in the lakes 
near the City of Mexico. ‘For many years these creatures were 
looked upon as a species of Perennibranchiate, under the generic 
name of Siredon (S. axolotl, S. pisciformis, S. mexicanus, etc.), 
although Cuvier suspected that they were but the larve 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 crea- 
tures were not larval, but a true Perennibranchiate species. 
But to the general surprise several of these young Axolotls gradu- 
ally 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.” * 
It was this discovery that first gave the hint that the axolotl 
is a larval form. At Weismann’s suggestion Mlle. Marie v. 
Chauvin tried to bring about this change artificially, and suc- 
ceeded in discovering the necessary conditions. The axolotls 
1 Gadow, H., “Amphibia and Reptiles,’ The Cambridge Natural History. 
2It appears that other observers had already recorded similar conditions. 
Filippi in 1861 found tritons sexually mature but without the adult form, and 
Jullien found four female larve of Lissotriton punctatus with mature eggs, two 
of which were laid. 
