490 BENNET M. ALLEN 
Bufo. These experiments clearly demonstrate that the thyreoid 
glands exert a marked influence upon development—an excess 
of thyreoid (feeding) accelerating metamorphosis and the re- 
moval of the thyreoid gland producing decided retardation of 
metamorphosis. It is thus seen that these two methods of 
investigation give results that are fully corroborative of one 
another. 
There are certain points in the solution of this problem that 
can be best attacked by a study of the normal relation of these 
glands to metamorphosis. It was shown in Rana pipiens that 
the thyreoidless and normal control tadpoles show no differences 
up to the time when the limb buds begin to develop, but as soon 
as these have made their appearance in the thyreoidless tad- 
poles they show a marked retardation in growth, while the body 
continues to grow in size. As far as our observations go, it was 
found that all somatic features of tadpole development were 
thus retarded, the gonads alone being unaffected (Allen, Rogers 
and Terry, 718). In order properly to interpret investigations 
of this kind, it is necessary to have definite data regarding the 
normal relation of the thyreoid glands to development. 
The development of the thyreoid glands of Anurans was 
described by Goette in Bombinator. A paper by W. Miller 
gave the first complete account of the process. This was worked 
out in Rana temporaria. Miller found that the thyreoid devel- 
oped from a ventral downgrowth of the floor of the pharynx. 
This anlage was at first unpaired, but later became divided into 
two parts by the development of the copula of the hyoid car- 
tilage. He traced the further development of the gland from 
a condition of a solid mass to a looser texture, accompanied by 
the disappearance of the pigment cells characteristic of the early 
stages, and he found later that it consists of a network of twisted 
cords which are surrounded by looser connective tissue. He 
traced the development through later stages in which vesicles 
had developed. These were composed of a single layer of epi- 
thelium and filled with colloid. He found that in young frogs 
immediately after metamorphosis, the thyreoid gland is wholly 
made up of the vesicles containing colloid. 
