168 
structural changes of the thyroid, setting in at an early period and 
being followed by extenuated or abolished function. Consequently 
the neotenic condition has to be interpreted as a hypothyreosis. 
The earliest stages of development of the organ offer no spec¬ 
ial interest. Individuals measuring 5 cm in lenght show cylindric 
epithelium and scarce colloid matter (pi. IV, fig. 1); whereas those 
measuring 9—10 cm have follicles distended by colloid matter and 
low cubic epithelium often extensively flattened (fig. 2). As a similar 
fiattening of the cells in spots also — though to a small extent — 
may be seen in larvæ of other urodela, in possession of a nor¬ 
mal metamorphosis (Åmblystoma punctatum (fig. 9 & 10), Triton 
punctatus , T. cristatus), it may admit of doubt whether this pheno- 
menon has to be interpreted as anything else than the result of a 
possibly transient pressure. In axolotls aged about Vs— 3 /* year 
— the very age at which metamorphosis in rare cases sets in 
spontaneously and at which v. Chauvin’s metamorphosis experi- 
ment sometimes may be successful — important changes of the 
thyroid take place. The organ will be found somewhat varying 
in size. The follicles are rather large and the epithelium being 
as a rule tall and cylindric (fig. 3) is subjected to proliferation. 
The epithelium rather regularly to a smaller or greater extent is 
flattened even to a very high degree (fig. 4), this being especially 
the case in the adjoining parts of neighbouring follicles. This fiat¬ 
tening process, displaying the character of an atrophy, develops 
further with the result that histological features, as given in fig. 5 
or 6, are regularly met with in animals aged 3 /4 — 1 year or still 
older. In the case of fig. 6 the epithelial layer was but membran- 
aceous. Coincident with this progressive epithelial atrophy are 
dilatation and partial coalescense of follicles, together with a quali- 
tative change of the colloid matter which will become liquid and 
less eosinophilic. The above facts concerning the structure of the 
thyroid have been demonstrable in specimens originating in four 
different strains of axolotls, and so they must be looked upon as 
features characteristic of the species. 
In larvæ of Åmblystoma punctatum approaching the juncture, 
fit for the setting in of the metamorphosis (4,5 cm in length), the 
thyroid is found to be rather small, composed of a restricted num- 
ber of roundish follicles equipped with cubic epithelium and con- 
