II.] THE COMMON FROG. 15 



In winter the Frog passes into that torpid state 

 known as hibernation, as is the case with our bats, 

 hedgehogs, and some other beasts. Its season of 

 torpidity is generally passed by it buried in mud 

 and at the bottom of water, and great numbers of 

 individuals may be dug up in winter all clustered 

 together. 



In spring the frogs again congregate for the pur- 

 pose of oviposition in the month of March, at which 

 period their well-known croaking makes itself heard, 

 and though in itself unmelodious, possesses a certain 

 charm through its association with the vernal outburst 

 of nature. 



When first laid, the Frog's eggs are little round dark 

 bodies enclosed in no solid shell or case, but in a 

 small glutinous envelope. The latter quickly swells 

 in the water, so much so that the '^ spawn" comes to 

 have the appearance of a great mass of jelly through * 

 which dark specks (the yolks of the %%^ are scattered. 

 Each ^%^, when microscopically examined, m^ay be 

 seen to undergo a process of yolk subdivision and 

 cleavage till a mulberry-like mass is formed. Upon 

 this soon appears the "primitive groove/' which 

 forms a canal and develops beneath it a ''chorda 

 dorsalis " according to the pr.ocess which has been 

 already stated to be common to the whole of the 

 Vertebrata. 



Gradually the embryo assumes the form of a young 

 tadpole, and is provided with a pair of little "holders" 

 (or organs for adhesion) just behind the mouth, with 

 six openings on each side of the neck (Fig. 2, cl^-cl^), 

 and with a pair of rudimentary external gills (Fig. 2, 



^'i 



