Electrocoagulation lesions involving 70\p=n-\100% of the hypophysis of 17 foetal lambs were made at various times between 93 and 136 days of gestation. Foetal growth and development after the operation was compared with that in a group of seven control lambs comprising three intact littermates of operated foetuses and four sham-operated foetuses. Hypophysial damage was found to be associated with retardation of somatic development which was most marked when the operation was performed early in gestation. Retardation was particularly evident in epiphysial development. Hypoplasia of the adrenal cortices, the interstitial cells of the testis and the thyroid glands confirmed the assumption made by previous workers based on decapitation experiments, that the foetal hypophysis exerts a trophic influence on the development of these endocrine organs. Reduced liver glycogen content and plasma glucose levels indicated a disturbed carbohydrate metabolism. Abnormal subcutaneous fat deposition suggested disorders in fat metabolism. Delayed involution of haemopoietic tissues, hypoplasia of peripheral lymphoid tissue and reduction in the number of mast cells in the skin were also noted. Structures of ectodermal origin were relatively unaffected by hypophysial lesions. It is concluded that the hypophysis of the foetal lamb subserves many of the functions attributed to it during extrauterine life.
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