STRUCTURES in the cystic breast composed of cells resembling those of the axillary apocrine sweat glands (hidradenocytes) were noted by several German pathologists.1 These cells are analogous to the occurrence of intestinal epithelium within the mucosa of the stomach or gallbladder. The peculiar odor of the content of certain cysts of the breast lined by these cells is similar to that of axillary sweat.
Phylogenetically, the milk glands of mammals are to be regarded as modified sweat glands. The basic structure of the mammary gland is a system of branched ducts, the ramifications of which vary greatly in response to endocrine stimuli. In the resting gland the ducts are lined by a double layer of cells. The blind cords constituting the terminal end pieces are frequently surrounded by stellate-shaped cells which are continuous with the outer layer of cells lining the ducts. These outer cells are collectively