302 ZOOLOGY SECT. 



A very interesting case of pwdogenesis is furnished by the Axolotl 

 (Amblystoma tigrinum). This animal frequently undergoes no 

 metamorphosis, but breeds in the gilled or larval state (Fig. 970). 

 But under certain circumstances the gills are lost, the gill-slits 

 close, and a terrestrial salamandrine form is assumed. It is to 

 the branchiate stage that the name Axolotl properly applies ; 

 before the metamorphosis was discovered its connection with 

 Amblystoma was not suspected, and it was placed in a distinct 

 genus, Siredon, among the Perennibranchiata. 



Segmentation of the egg in the Anura and Urodela is always com- 

 plete but unequal. In Pipa and Alytes there is a large quantity 

 of food-yolk, and the developing embryo lies on the surface of 

 a large yolk-sac. In the Gymnophiona the eggs, which are singularly 

 like those of a Bird, are of large size and segmentation is partial, 

 the formation of segments at the pole of the egg opposite that at 

 which the formation of the embryo begins only taking place at 

 the stage of gastrulation : the embryo is coiled over the surface 

 of the yolk as in the Trout. 



Distribution. The Urodela are almost exclusively Palsearctic 

 and Nearctic forms, occurring in North America, Europe, Asia, and 

 North Africa : a few species extend southwards into the Neotropical 

 and Oriental regions. The Gymnophiona, on the other hand, are 

 mainly southern, occurring in the Neotropical, Ethiopian, and 

 Oriental regions, but are absent in Australasia and the Pacific 

 Islands. The Anura are almost universally distributed, and are 

 abundant in all the greater zoo-geographical regions : they are, 

 however, very meagrely represented in New Zealand and are absent 

 in most Oceanic islands, a fact due to the fatal effects of salt water 

 upon the eggs and embryos of Amphibia as well as upon the adults. 



Remains of Stegocephala are found in considerable abundance 

 from the Carboniferous to the Trias, and one genus extends 

 into the Lower Jurassic, after which period the order apparently 

 became extinct. The Urodela and Anura are not known until 

 the Eocene, and no fossil remains of Gymnophiona have been 

 found. 



Mutual Relationships. The perennibranchiate Urodela are 

 undoubtedly the lowest of existing Amphibia ; they lead up, through 

 such forms as Amphiuma, with persistent gill-slits but deciduous 

 gills, to the Land Salamanders, in which a purely terrestrial form is 

 assumed. The Stegocephala exhibit a parallel series of modifications, 

 some of them being perennibranchiate, others caducibranchiate. 

 Their skull is more complex than that of the Urodela, but their 

 vertebral column never reaches the same degree of specialisation as 

 that of the Land Salamanders, and in some cases shows a lower 

 grade of organisation than in any existing Amphibia. Both in 

 their skeleton and in the distribution of their lateral sense-organs 

 they show some affinity with the Crossopterygii. The Anura 



