, thyroid
has been used to transform one into the other. Thus the occasional
change of a Mexican axolotl, a purely aquatic newt, breathing through
gills, into the amblystoma, a terrestrial salamander, with spotted
skin, breathing by means of lungs, has long been known. Feeding the
axolotl on thyroid gland produces the metamorphosis very quickly, even
if the axolotl is kept in water. In the reptile house at the London
Zoological Gardens full-grown examples of the common black axolotl and
the pretty white variety are exhibited. Some are nearly three inches
long. Alongside are shown several examples of the amblystoma stage,
produced in one of the laboratories of Oxford University and at
the gardens by thyroid feeding. A variation of the thyroid in the
direction of increased secretion was probably responsible for the
first land animals.

THYROXIN, SECRETION OF THE THYROID

Under the microscope, as in the test tube, the thyroid shows
remarkable and unique features. Closed spherules lined by a single
layer of cells enclosing a gelatinous material known as colloid, which
stains deeply with acid dyes, comprise the units of its architecture.
Essentially, it may be pictured as a series of jelly bubbles secreted
by outlying cells.

A relatively high percentage of iodine is the unique distinctive fact
in its chemistry. Discovered by Baumann in 1895, the presence of the
element has focused the intelligence of chemists upon the gland,
with the consequent demonstration of arsenic also in it. It was soon
manifest that the secretion of the gland was dependent upon the
iodine content for its activity. Active extracts of the thyroid like
thyreoglobulin and iodothyrin were proven to contain iodine, and to
become inactive when the iodine was removed. Efforts to isolate the
iodine containing active principle in pure form were fruitless until
the work of Kendall at the Mayo Foundation. He obtained it as a white,
finely crystalline, odorless and tasteless substance, heat stable,
and analyzable. The free form separates as a sheaf of fine needles.
Kendall at first called it the a-iodine compound, then named it
thyroxin.

There are other internal secretions of the thyroid, with a function of
their own, that have no iodine. But they are secondary, and obscure.
Thyroxin is accepted today as the purified internal secretion of the
thyroid because all the effects of the whole gland may be elicited
with it. Thyroxin produces results with doses am