THE RECAPITULATION THEORY 329 



the extraordinary modifications presented by the 

 embryonic and larval respiratory organs of Amphi- 

 bians. Confining ourselves to those forms which 

 do not lay their eggs in water, and in which con- 

 sequently development takes place within the egg, 

 we find that Ichthyophis and Salamandra have 

 three pairs of specially modified external gills. 

 Nototrema has two pairs ; Alytes and Typhlonectes 

 have only a single pair, which in the latter genus 

 take the form of enormous leaf-like outgrowths 

 from the sides of the neck. In Hylodes and Pipa 

 there are no gills, the tail acting as the larval 

 respiratory organ ; and in Rana opisthodon, accord- 

 ing to Boulenger, larval respiration is effected by 

 nine pairs of folds of the skin of the ventral surface 

 of the body. 



Most of these extraordinarily diversified organs 

 are clearly secondarily acquired structures ; it is 

 possible that they all are, and that external gills, 

 as was suggested by Balfour for Elasmobranchs, 

 are to be regarded as embryonal respiratory organs 

 acquired by the larvae and of no ancestral value. The 

 point however cannot be considered settled, for on 

 this view the external gills of Elasmobranchs and 

 Amphibians would be independently acquired and 

 not homologous structures, a view contradicted by 

 the close agreement in their relations in the two 

 groups, as well as by the absence of any real 

 break between external and internal gills in Am- 

 phibians. 



It is well known that the frog and the newt 

 differ greatly in important points of their develop- 



