BELA.TIOK OF METAMORPHOSIS TO FERTILITY. 



121 



on the contrary, which pass through a metamorphosis always arise 

 from eggs of relatively small size, are hatched in an immature con- 

 dition as larvse, and obtain independently, by their own activity, the 

 materials which have been withheld from them while in the egg, 

 but which are necessary for their full development. The number 

 of embryos produced in the case of a direct development is, in 

 proportion to the total weight of the material applied by the mother 

 for reproductive purposes, far smaller than in the case of a develop- 

 ment with metamorphosis. The fertility of animals whose young 



a 



FIG. 112, Later stages in the development of Pelobates fuscus. a, larva without limbs 

 with well developed tail ; b, older larva with hind limbs ; a, larva with two pairs of limbs ; 

 d, young frog with caudal stump ; e, young frog after complete atrophy of tail. 



undergo a metamorphosis, or, in other words, the number of offspring 

 produced from a given mass of formative material, is increased to 

 an extraordinary degree, and has, in the complicated relations of 

 organic life, a great physiological significance, though systematically 

 it is of little importance. 



Some time ago it was attempted to explain these indirect meta- 

 morphoses, in which both processes of reduction and new development 

 take place, as the result of the necessity which the simply organized 



