Changes in the Life-cycle 315 



Influence o] the Environment on the Time oj 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 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 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. 2 At Weismann's suggestion Mile. 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. 



2 It 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 larvae of Lissotriton punctatus with mature eggs, two 

 of which were laid. 



