I. Stages are described in the development of the mammae of male rabbits responding to regular injections of oestrogen and progesterone.
2. It is suggested that the extra mammary lobes which develop under this treatment are derived from hair follicles and sebaceous glands by the loss of the hairs from the follicles and the proliferation of sebaceous glands and ducts.
3. This may throw light on the phylogenetic origin and homology of the mammary gland.