B.H. Willier (Analysis of Development, W.B. Saunders, Philadelphia, 1955, pp. 574-619) hypothesized that the early differentiation and initial function of the glandular components of vertebrate endocrine axes are autonomous and that functional interdigitation of these glands occurs later in developmental time. The present review summarizes findings that indicate that the hypothalamus, pituitary, and thyroid glands of the developing chick embryo synthesize their appropriate hormones early in development, i.e., before day 5.5 of incubation. Observations also indicate that it is not until the days 10.5-11.5 embryonic period that the feed-forward and feed-backward components of the adenohypophyseal-thyroid or the hypothalamo-adenohypophyseal-thyroid axis mature. Evidence is also presented for a midgestational beginning of hypothalamic regulation of adenohypophyseal-thyroid unit function.