148 The Microscope. 



The young tadpole's gills begin to be sufficiently 

 transparent for microscopic observation when it is 

 about the size represented in No. 24, h. At that time 

 its tail is opaque, and the tail-fin or web very slightly 

 developed. The same tadpole's gills will on the 

 following day be seen to have grown, and during 

 about two days more they will show well, the tail- 

 fin also becoming broad and transparent, and circula- 

 tion showing well in every part of it. After this 

 time the gills begin to shrink in size (a process called 

 "absorption"), and can be observed (as I found) 

 during about six days in the same tadpole. No. 24, i, 

 shows the tadpole on the last day in which the 

 branchige are worth examining. The interesting 

 spectacle can, however, be observed each spring for 

 about three weeks in all in any locality, because 

 families of tadpoles in various stages of advancement 

 can be found in different ponds. By the time the 

 tadpole's gills have disappeared (being for a while 

 concealed under gill- covers, somewhat like those of 

 fishes) the circulation in the tail-fin, d, No. 24, has 

 become a most beautiful object, and it continues to be 

 so till after the tadpole has developed all its feet, and 

 assumed much of the angular outline of the frog. 

 After this time the tail begins to shrink in size, 

 becoming absorbed just as was the case with the gills. 

 The frog (as the creature must now be called), is a 

 droll-looking object, when leaping with about a 

 quarter of an inch of tail remaining; and in a few 

 days even this remnant will have dwindled away. 



The circulation seen in the web of a tadpole's tail 



