THYREOID GLANDS OF BUFO 497 
The tadpoles selected from a jar containing many hundred 
were closely matched in six representative stages with ten speci- 
mens of each stage. The measurements of lengths of hind leg, 
body, and total length give a basis for comparing these lots. 
Of these criteria the most constant is the length of the hind 
legs. These show a continuous growth, while the total length 
and body length are modified by the process of metamorphosis, 
the body showing distinct reduction for a time. The stages 
chosen may be described as follows: 
I. Hind-limb buds very small, the longest showing but faint 
indications of differentiation into parts. No evidence of fore 
legs (fig. 1 a). 
II. Total length and body length increased. Hind limbs 
showing differentiation into parts. Toes well differentiated. 
III. Hind limbs decidedly larger than in preceding stage. 
Continued increase in total and body length. Fore limbs formed 
beneath the skin, but not yet broken through (fig. 2 a). 
IV. Continued increase in length of hind limbs. Fore limbs 
through the skin. Slight decrease in total length and body 
length. 
V. Continued increase in size of limbs. Slght increase in 
body length, but marked decrease in tail length. 
VI. Completion of metamorphosis (fig. 3 a). 
In the main there is little need of comment upon the figures in 
the accompanying tables. In stage I (fig. 4) there is little 
colloid present in the follicles of the thyreoid glands and many 
of the follicles are not yet formed, being represented merely by 
small scattered masses of cells. They lie all in one plane at 
this time, except in a few cases where they are beginning to 
arrange themselves in two layers. This process is completed in 
stage II, where the thyreoid glands show a distinct increase in 
size and in the number and size of the component follicles (fig. 5). 
This is continued through later stages. It will be seen that 
there are many cases where the volume of the thyreoid glands is 
not proportional to the relative length of the legs or of the body. 
While this is true in a comparison between the members of group 
I and group II (with the single exception of II h), there are no 
