The histogenesis of carcinoid tumours of the rectum.

;Carcinoid' tumours of the rectum have been described which are classified on histological appearances into three main varieties: ;true' carcinoid or argentaffinoma, ;atypical' or non-argentaffin carcinoid, and ;composite carcinoid. The histogenesis of these tumours is discussed and it is suggested that non-argentaffin carcinoids of the rectum have an essentially similar histogenesis to certain tumours of the lung, pancreas, and gastrointestinal tract. The difference in growth, pattern, and staining reactions are a reflection of differing directions and levels of differentiation of the parent epithelium. The functional implications of this hypothesis are discussed.