70 
L. Kaufman: 
Water, with adequate quantities of air, delays metamorphosis; 
absence of oxygen in water or the impossibility of breathing (as in 
tadpoles put on land) accelerates transformation. This may be ex¬ 
plained in two different ways: either by admitting that the impos¬ 
sibility of branchial respiration stimulates the function of the lungs 
and that the changes of blood circulation connected therewith cause 
degeneration of the gills and fin. or by supposing that deficiency 
or unsufficient supply of 0 2 stimulates the thyroid gland physiolo¬ 
gically. The first hypothesis does not appear satisfactory; it does 
not explain the loss of weight of the larvae undergoing metamor¬ 
phosis, nor the fact that not only the gills and the fin. but the or¬ 
ganism in general is affected by histolytical processes. The second 
hypothesis is confirmed by some observations. The absence of the 
thyroidea in animals causes a decrease in digestion of albumen up 
to 50%; on the other hand, pathological hypertrophy of this organ 
produces a larger excretion of nitrogen. Digestion of albumen is 
also accelerated by deficiency of oxygen (suffocation, loss of blood, 
mountain air, poisons). Mannsfeld and Müller endeavoured to 
ascertain whether this deficiency affects the plasma directly or whe¬ 
ther we have here to do with an intensified function of the thyroid 
gland. They studied changes of albumen digestion occurring when 
oxygen was wanting, both in normal rabbits and in specimens with 
removed thyreoidea. Under these conditions normal animals always 
showed greater metabolism of nitrogen, whereas specimens deprived 
of thyroid gland did not react upon the stimulus. Deficiency of 0 2 
therefore is in the first rank a physiological stimulus to the thyr¬ 
eoidea. 
That oxygen deficiency stimulates secretion of the thyroid gland 
in the larvae of amphibians, thus leading to metamorphotic proces¬ 
ses, may be inferred from studying the duration of larval life and 
the behaviour of the thyroid gland in animals kept both in thor¬ 
oughly aerated water and in water with unsufficient amount of 
oxygen. I hope to investigate this question in a future paper. 
6. Summary. 
1. Feeding the larvae of Siredon pisciformis on adequate doses 
of thyroidine preparations with meat, we obtain vivid terrestrial 
forms (Amblystoma mexicanum). 
