102 THE AMERICAN NATURALIST. [Vor. XXXIV. 
tail, and tadpoles hang in the water with the tail at least 20° 
lower than the head. Many of the older ones seem no heavier 
than water. As soon as tadpoles begin to depend on food 
taken into the alimentary canal from without, the canal may 
become filled with air or gas, so that the animal floats belly up. 
This condition is not permanent, for I have isolated such floating 
tadpoles and they regained the bottom in the course of a day. 
I have seen a tadpole force from its mouth as many as twelve 
bubbles of gas. After attaining the length of 10-11 mm., the 
specific gravity of the tadpole depends much on the condition 
of the animal and on the development of the lungs. The 
refuse of the food in the alimentary canal, which consists of 
partially digested green water plants and many diatoms, may 
aid the animal in sinking. It certainly helps move back the 
center of gravity of the animal from the head region toward 
the tail. 
H. ' 
I have found the specific gravity of four series of tadpoles — 
two in the spring of 1898 and two in the spring of 1899. In 
each instance one series was the young of Rana silvatica, the 
other those of Bufo lentiginosus. 
TABLE IV. — Speciric GRAVITY OF FROG AND Toap TADPOLES. 
Rounp 
LENGTH ID oe — - E om a 
EN IN MM Eca. 23 4-5 6-7 8-9 | ro-11 | 12-13 | 14-15 |16-16+ 
Rana silvatica, 1898 — | — |r022|1.013| — |I.013| 1.008 | 1.005 1.0055 
Rana silvatica, 1899 — |I-111/1.11—|1.050| 1.042| 1.028| — | 1.010|1.0052 
Bufo lentiginosus, 1898 | 1.038 |1.037| 1.037, — |1.022|1.013|1.013|1.010| — 
Bufo lentiginosus, 1899 | 1.037 | 1.037 | 1.030) 1.022| 1.017| — |1.016|1.016| — 
This table calls for a few remarks. 
I have been able to get no good determinations oe the specific 
gravity of the eggs of the frog, for unless the egg envelope is 
removed, the specific gravity is too high; and if the envelope is 
removed from the youngest eggs, they break badly in the solu- 
tion. The toads for the two years correspond fairly well with 
