A series of grafting experiments was conducted to determine pituitary origins prior to brain tube closure in Xenopus laevis. Extirpation experiments indicated that the ventral neural ridge (VNR) tissue of stage-18+ embryos was essential for pituitary development. Bolton–Hunter reagent was used to label stage-18+ VNR tissue with 125I, and this tissue was then returned to the donor and its subsequent ontogenesis followed. Labelled tissue was ultimately found in the ventral hypothalamus, the ventral retina, and the anterior pituitary. Using immunocytochemical techniques with antisera to adrenocorticotropin (ACTH), it was found that some of the VNR-derived cells were corticotropes. A region of the nucleus infundibularis which was radioactive labelled also gave ACTH-positive immunoreaction. This might indicate that some ACTH containing neurones of the hypothalamus are VNR in origin. We suggest that stage-18+ VNR is the site of attachment of brain and anterior pituitary ectoderm. Part of this adherence point is eventually incorporated into the anterior pituitary and will form corticotropes. It is concluded that the ventral retina, the preoptic region of the hypothalamus, some hypothalamic ACTH-immunoreactive cells, and the most anterior portion of the adenohypophysis are all ventral neural ridge in origin.
