Abstract
THE phenomenon of molt has long been observed and studied, yet the physiological mechanisms causing molt remain obscure. Hill (1935) observed that the hypophysectomized cock began to molt two to four weeks after the operation and remained in a perpetual state of molt. It has been thought that the circulating level of thyroxin may be an important factor in controlling the onset of molt. This view is based on investigations showing that when thyroid activity is greatly increased, as by feeding desiccated thyroid powder, thyroprotein, or by injecting thyroxin, molting can be induced or the rate of feather growth of young chickens increased, and upon investigations showing that the feeding of thiouracil will delay the onset of the natural molt (Sturkie, 1954). These treatments result in much wider variations in thyroidal activity than would probably occur naturally, hence, it seems improbable that this could be the sole cause of the …
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