SummaryTumours and cysts of the pituitary region may be simple or complex in form.The complex structures, usually para pituitary in position are commonly regarded as having an origin directly from such archaic structures as the hypophyseal duct or less clearly recognizable “cell residues.”The assumption that such tumours or cysts arise from special tissues is due to the view that only these possess the potentialities for production of several tissues and that such capacity is not present in more adult structures.This assumption is misleading on two accounts: (i) the cysts and tumours are not in reality as complex as cursory examination would suggest, and (ii) investigation shows that powers of differentiation greater than those previously recognized are, in effect. present in adult tissues.The various tissues that are seen in these complex structures develop by changes in cells previously present; thus, the cyst or tumour does not arise like Minerva, fully panoplied, from the brow of Jove but develops by gradual differentiation of proliferating cells in the same manner as is seen in the formation of normal tissues or organs.In a cyst, for example, there first appears a simple epithelium which may be in part ciliated; this may become pseudostratified or stratified (the former may be ciliated and the second may show keratin formation). The squamous epithelium may then form a basal‐type epithelium with calcifiable homogeneous secretion. Lymphocytic accumulation and cartilage formation occurs and, in areas of excessive keratinous formation, cholesterol accumulation may be found. When all these various tissues and materials are found together the structure becomes one of considerable macroscopical and histological complexity.The close relation of such complex tumours and cysts to simple ones is thus clear and it is unnecessary to regard the essential progenitor of such structures as a “totipotent” cell or a cell of some special kind. The complex cyst or tumour is merely a variant of that which, because it closely resembles the tissues commonly observed in the part, is usually regarded