232 METAMORPHOSIS AND 



met in various ways. In the South American tree- 

 frogs of the genus Nototrema the eggs are developed 

 in a dorsal pouch of the skin of the female, and 

 within this pouch the respiration of the embryo is 

 carried on by a membranous expansion of the second 

 and third external gills on each side. In the Reptilia 

 the bladder is expanded for the same function, and 

 absorbs oxygen and gives off carbon dioxide through 

 the pores of the shell. It is impossible to reconcile 

 the conception of mutation with the adaptive re- 

 lation between this allantois and the expulsion of the 

 egg enclosed in a shell on land. The transition 

 probably came about gradually from the deposition 

 of the eggs in moist places but not in water. In the 

 midwife toad (Alytes obstetricans) the male carries 

 the eggs about attached to his legs, respiration is 

 effected by enlarged external gills, and the larvae 

 are hatched in water. In the ancestral reptiles 

 external gills may have helped at first, until by the 

 enlargement of the bladder they were rendered un- 

 necessary. In all such cases the absorption of oxygen 

 must be regarded as the stimulus which caused the 

 enlargement of the respiratory membrane. As the 

 allantois could not be absorbed or retracted again 

 into the abdomen, the umbilicus was evolved that is 

 to say, the scar formed by the union of the folded 

 edge between the body wall and amnion surrounding 

 the stalk of the allantois. It would be difficult for a 

 mutationist to explain how a mutation should affect 

 the development of the cloacal bladder to such an 

 enormous degree, just when it was required for 

 embryonic respiration, and cause the sides of the body 

 to unite ventrally at the time of hatching, cutting off 

 the allantois and the amnion. 



