Effect of Thyroxin on Growth Rate and Carbon Dioxide Production of Chick Embryo.

The data presented here represent the attempt at studies made to analyze the influence of thyroxin upon the pre-natal development of the chick. The technique employed was the same as previously described by the writer 1 for injecting substances into the air sac of the incubating hen's egg. The thyroxin was dissolved in dilutions of sterile distilled water just alkaline (NaOH) to litmus so that the required dose was contained in 0.25 cc. The controls were injected with the same amount of alkaline water.

Experiments were made to determine the appropriate dosage that would be within physiological limits. It is necessary to explain that before the sixth and eighth days of incubation the albumen lies between the air sac and the developing chorio-allantois. Thus an injection placed in the air sac previous to this age would be diluted by its absorption into the albumen. A test dose of 1/300 mg. of thyroxin was given before the start of incubation. This dose proved toxic, so the experiment was repeated giving 1/600 mg. before incubation, and repeating the dose on the thirteenth day. None of the chicks hatched. Then the experiment was again repeated giving only 1/600 mg. on the fourth day with the result that 17 hatched out of 24 incubated eggs. These results indicated that the dose of thyroxin injected into the albumen to be below toxicity must not be over 1/600 mg.

Ey the sixth or eighth day the albumen is crowded toward the vegetative pole and the respiratory vascular network of the chorioallantois comes into direct contact with the air sac membrane. From this time until hatching absorption takes place directly into the Mood stream. Advantage was taken of this in other experiments and results showed that at this time, the maximum dose compatible with hatchibility is in the neighborhood of 1/40,000 mg. of thyroxin.