Landauer1, 2 has found that certain morphological abnormalities may be produced by injecting insulin into the yolk of 0-5-day chick embryos. In the course of an investigation into the carbohydrate metabolism of insulin-treated embryos we have found that a persistent hypoglycemia is produced by insulin applied in this manner. This phenomenon is interesting because the hormones usually related to insulin activity are probably not present during the first 7-8 days of embryonic life.The only certain information about the pituitary gland concerns the onset of cytological differentiation. Rahn3 has shown that acidophils and basophiles do not appear before the 10th day (though both are preceded by a general basophilic tendency starting on the 8th day). The adrenal cortex (Okuda) is not formed until the 6th day; when cortin first appears is uncertain. Islets of Langerhans are not differentiated until 8-9 days and detectable amounts of insulin do not appear until the 11th day. Needham has emphasized that there is ...