268 INTRODUCTION TO ZOOLOGY. 



(Fig. 211), the flesh of which is regarded as an article of 

 luxury by the inhabitants of the city of Mexico, near to which 

 it is taken. It is, however, to the latter group that we wish 

 more particularly to call attention; for in the Frogs, Toads, 

 and Newts of these countries, we have the opportunity of 

 watching the successive steps by which they become fitted 

 for breathing air, instead of continuing to use an apparatus 

 adapted, like that of fishes, for aquatic respiration only. 



Fig. 211. AXOLOTL. 



Let us give our attention, in the first instance, to the 

 changes which take place in the common Frog (Rana tempo- 

 raria). The eggs are deposited at the bottom of a pool of 

 water, each egg consisting of a black centre, surrounded by a 

 covering of glutinous matter. This covering absorbs water; 

 the mass swells, so that the central portions appear like black 

 dots, separated from each other by a transparent jelly; and 

 owing, as Professor Bell states, to some partial decomposition, 

 and the consequent disengagement of a gas, the entire mass 

 becomes lighter than the surrounding water, and rises to the 

 surface. It is in this stage that we have taken some of the 

 spawn, and kept it in glass vessels for the purpose of watching 

 the subsequent changes, which are much influenced by the 

 temperature of the apartment. When the little Tadpole has 

 burst from its prison, the gills begin to develope themselves, 

 and increase rapidly in size until they attain their greatest 

 development. They are now objects of singular beauty when 

 viewed through the microscope; for such is their transparency 

 that the course of every globule, as it passes up or down the 

 main stem, or enters the inlets presented by each leaf, is dis- 

 tinctly visible. The delight with which this spectacle is 

 regarded by children, and the interest they henceforward take 

 in the previously despised Tadpole, are matters of which we 

 can speak from personal experience. This period of expan- 



