CONCERNING TADPOLES 197 



In several species of frogs the eggs are carried by the 

 female upon her back. In Goeldi's tree-frog these eggs, 

 few in number and large, are kept in position partly by 

 their own adhesive nature, and partly by a fold of skin 

 which runs round the lower region of the back, forming a 

 low parapet to support the precious burden. Now mark 

 what follows. From this we pass to a nearly related Vene- 

 zuelan species {Nototrema pygmesum), in which the walls 

 of this parapet rise up on either side and arch over the 

 back till they nearly meet, leaving but a mere slit between 

 them. Thus the eggs are enclosed in what is practically a 

 pouch. By an easy transition the formation of a complete 

 pouch could be accomplished by the actual closing of the 

 slit and the fusion of the edges thus brought together. 

 This completion of what was foreshadowed in Goeldi's 

 frog is found in the pouched frog {Nototrema marsupia- 

 tum), shown in the adjoining illustration. 



There are several species which thus bear the eggs, 

 which may range in number from seven to one hundred. 

 Where the eggs are few in number, be it noted, they are 

 large, containing a considerable amount of food-yolk, and 

 in such cases the larval stage is passed within the egg. 

 In some of the cases, at any rate, these larvae have been 

 carefully studied, and it has been found that the mechanism 

 for ensuring an adequate aeration of the blood, that is to 

 say, for breathing, is somewhat remarkable, inasmuch as 

 the external gills, instead of assuming the characteristic 

 feathery shape, take the form of long stalks surmounted by 

 a convolvulus-like cup, which is richly supplied with blood- 

 vessels. 



The celebrated Surinam toad {Pipa americana), one of 

 the ugliest members of its tribe, presents another, and if 

 possible, still more extraordinary variation in the method 



