116 METAMORPHOSIS. 



tadpoles of Toads are the smallest, Pseudophryne australis excelling in 

 this respect ; those of Pseudis are the largest known. 



The external gills reach in certain forms, which are hatched in late 

 larval stages, a very great development. It seems however that this de- 

 velopment is due to these gills being especially requii-ed in the stages before 

 hatching. Thus in Alytes, in which the larva leaves the egg in a stage 

 after the loss of the external gills, these structures reach in the egg a very 

 great development. In Notodelphis ovipara, in which the eggs are earned 

 in a dorsal pouch of the mother, the embryos are provided with long vesi- 

 cular gills attached to the neck by delicate threads. The fact (if confirmed) 

 that some of the forms which are not hatched till post-larval stages are with- 

 out external gills, probably indicates that there may be various contrivances 

 for embryonic respiration' ; and that the external gills only attain a great 

 development in those instances in which respiration is mainly carried on 

 by their means. The external gills of Elasmobranchii are probably, as 

 stated in a jjrevious chapter, examples of secondarily developed structures, 

 which have been produced by the same causes as the enlarged gills of 

 Alytes, Notodelphis, etc. 



Urodela. Up to the present time complete observations on the 

 development of the Urodela are confined to the Myctodera''. 



The early stages are in the main similar to those of the Anura. 

 The body of the embryo is, as pointed out by Scott and Osborn, ven- 

 trally instead of dorsally flexed. The metamorphosis is much less 

 complete than in the Anura. The larva of Triton may be taken as 

 typical. At hatching, it is provided with a powerful swimming tail 

 bearing a well-developed fin : there are three pairs of gills placed 

 on the three anterior of the true branchial arches. 



Between the hyoid and first branchial arch, and between the other 

 branchial arches, slits are developed, there being four slits in all. 

 At the period just before hatching, only three of these have made 

 their appearance. The hyomandibular cleft is not perforated. Stalked 

 suckers, of the same nature as the suckers of the Anura, are formed on 

 the ventral surface behind the mouth. A small opercular fold, deve- 

 loped from the lower part of the hyoid arch, covers over the bases of 

 the gills. The suctorial mouth and the provisional horny beak of the 

 Anura have no counterpart in these larvee. The skin is ciliated, and 

 the cilia cause a rotation in the egg. Even before hatching, a small 

 rudiment of the anterior pair of limbs is formed, but the hind-limbs 

 are not developed till a later stage, and the limbs do not attain to 

 any size till the larva is well advanced. In the course of the sub- 

 sequent metamorphosis lungs become developed, and a pulmonary 

 respiration takes the place of the branchial one. The branchial slits 

 at the same time close and the branchiae atrophy. 



1 In confirmation of this view it may be mentioned that in Pipa Americana the 

 tail appears to function as a respiratory organ in the later stages of development 

 (Peters). 



2 The recent observations on this subject are those of Scott and Osborn (No. 114) 

 on Triton, of Bambeke (No. 95) on various species of Triton and the AxolotI, and of 

 Clark (No. 98) on Amblystoma punctatum. 



