169 
taining ampie colloid matter (fig. 10). In specimens measuring 8 cm 
in length (terrestrial form!) the organ is made up of a consider- 
able number of roundish follicles with cubic epithelium and rich, 
highly eosinophilic colloid matter (fig. 8). In older individuals of 
about 14 cm the follicles are enlarged and partially amalgamated, 
the epithelium being lower (fig. 11). Finally in adult animals, aged 
several years (17 cm), the large, irregular follicles with highly flat- 
tened epithelial cells are filled with a semifluid colloid matter but 
slightly receptive of eosin staining (fig. 12). 
In some not quite full-grown specimens of Amblystoma tigrinum 
(fig. 7), opacum and microstomum conditions have been established 
wholly corresponding to those found in Amblystoma punctatum. 
The regressive changes of the thyroid in Amblystoma puncta¬ 
tum (and other Amblystoma species) are setting in late so pre- 
senting the character of senile changes, whereas in Amblystoma mex- 
icanum they initiate at an early age developing in irregular ways. 
II.-The Condition of the Thyroid in cases of Irregular Metamor- 
phosis in Bufo and Rana. 
In Bufo larvæ (pi. VI, fig. 13—16) the thyroid is composed of 
a small number of follicles, whereas in Rana larvæ (/?. arvalis (fig. 
17—18) and esculenta (pi. VI, fig. 19—20; pi. VII, fig. 24—26)) 
the organ is larger and the number of follicles greater. The epi¬ 
thelium is cubic or low cylindriform. During the metamorphosis 
vacuolation of the colloid matter appears; this change (resorption?), 
which also is found in urodela (Triton, Salamandra, Amblystoma), 
is not met with at earlier stages of development thus being, as it 
seems, connected with the metamorphosis. 
A. - Precocious Metamorphoses. The material examined includes 
tadpoles of Bufo vulgaris and Rana arvalis. 
a. — In a homogeneous culture consisting of several hundreds of 
tadpoles ( Bufo vulgaris ) 3 individuals displaying precocious 
■metamorphosis were found (fig. A.: lower row; the corre¬ 
sponding Control specimens are figured in the upper row). 
The thyroids of the control specimens examined looked much 
the same (pi. VI, fig. 14), whereas in the three small four- 
legged individuals the thyroids were smaller, the follicles 
