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As far as gross structure is concerned the mammary glands of the monotremes consist of discrete lobules, in effect compound alveolar glands similar to those of the fully lactating marsupial. Ultrastructurally all mammary glands, prototherian, metatherian, and eutherian are identical, the alveoli consisting of a secretory epithelium invested by myoepithelium. The processes of formation and secretion of casein and milk fat are the same in the glands of all three subclasses of the Mammalia. In spite of close structural similarities the fatty acid complements of the milk triglycerides of the two monotremes, platypus and echidna, are different: echidna milk fat contains far more oleic acid than platypus milk does.