Neuroectodermal origin of avian hypothalamo-hypophyseal complex: the role of the ventral neural ridge.

Embryonic discs from White Leghorn chick embryos (presomite to 12-omite stages) were examined in serial transverse and longitudinal sections. Later stages, up to 21 days, were also examined. The following observations were made: (1) Lateral folding rather than head folding is the mechanism by which the avian embryo is delimited. The process takes place at two terminal points, one cephalic the other caudal. (2) Thickening of the ventral ectoderm around the tip of the presumptive anterior neuropore, beginning at the 4-somite stage, subsequently extends the dorsal neural ridge to form a ventral neural ridge. (3) The cephalic portion of the ventral neural ridge, extending from anterior neuropore to optic chiasma, is mainly incorporated into the alar plates of the diencephalon. (4) The caudal portion, extending from optic chiasma to the stomodeum, gives rise to Rathke's pouch and thus to the adenohypophysis. We conclude that the latter is to be regarded as of neuroectodermal rather than ectodermal (stomodeal) origin, and that some or all of the neuroendocrine nuclei of the hypothalamus are similarly derived from the neuroectoderm of the neural ridge. The hypothalamo-hypophyseal complex is thus to be regarded as a single rather than a composite entity.