INFLUENCE OF QUANTITATIVE THYROPROTEIN TREATMENT OF HENS ON LENGTH OF INCUBATION PERIOD AND THYROID SIZE OF CHICKS1

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Abstract
Offspring of hens fed supplemental thyroxine in the form of synthetic thyroprotein at levels of 0.02%, 0.04%, and 0.08% of the diet require longer periods of incubation and exhibit marked thyroid enlargement. The 2 phenomena occur concomitantly and appear to be rather directly associated with each other at the dosage used. The effects appear to be due to a reduction of maternal thyroid activity and a failure of the thyroidal substance in thyroprotein to be deposited in the egg. Inhibition of the hen''s thyroid results in the deposition of insufficient amts. of maternal hormone in the egg which is reflected in enlargement of the chick thyroid and delayed hatching. Release of inhibition of the hen''s thyroid when the hen is replaced on the control diet is followed by deposition of an oversupply of maternal hormone in the egg which is reflected in a reduction of the chick thyroid to subnormal size and by a tendency toward accelerated hatching. Evidence is presented which suggests that the secretory products formed by the enlarged chick thyroid as a result of uninhibited pituitary stimulation are stored and are ineffective. Delayed hatching may be due to: (1) reduced rate of embryonic development as a result of deficient maternal hormone, or (2) ineffective functioning of the chick thyroid during the last half of incubation.