THE COMMON FROG. 87 



fessor Quatrefages has well described these 

 changes : 



In this group we meet with both complete and incomplete 

 metamorphoses ; but here we find them marked by quite 

 different features from those among insects. The changes 

 do not appear to take place suddenly, nor is there anything 

 like the apparently torpid condition of the pupa. All the 

 transformations take place gradually, and as far as the ex- 

 ternal organs are concerned, the development may be closely 

 watched by the observer. 



The development of frogs presents another curious pheno- 

 menon. It is this : the young animal, after it has left the 

 egg, and before it has become a larva, is still in a semi- 

 embryonic condition. At this period the digestive tube and 

 its appendages are exceedingly rudimentary. The greater 

 portion of the body is filled by a large mass of yolk or 

 vitellus, inclosed by the skin, which has been formed for 

 some time ; and it is at the expense of this alimentary matter 

 that the development proceeds. 



The external characters are in keeping with the imperfect 

 condition of the animal at this period. The head is large, 

 and appears to be divided in two on the under surface, each 

 half being prolonged as a sort of process by which the animal 

 attaches itself to surrounding objects ; as yet there are no 

 traces of either eyes, nostrils, respiratory or auditory organs ; 

 and the belly, of an oblong form, is continued posteriorly as 

 a short tail bordered with a riband-like membrane. This 

 primitive condition, however, does not last long. About 

 the fourth day after birth, the head, which is now as long as 

 the body, has somewhat the appearance of a thimble ; the 

 mouth is provided with a pair of soft lips ; the nostrils, eyes, 

 and auditory apparatus have made their appearance ; the 

 head is separated by a deep groove from the belly, which has 

 assumed a spherical form, and from which spring a pair of 

 opercula, clothed with little branching gills ; and the tail 



