INTRODUCTION Normal onset of lactation In the majority of species investigated it would appear that oestrogen stimulates mammary duct growth, but that alveolar development requires luteal secretion. To this the cow and the goat are evidently exceptions, and perhaps this is not unconnected with the fact that they have been domesticated for milk production. A marked difference between the cow and the goat, which is perhaps rather singular, concerns the development of the mammary gland. Among goats of high milking strains, though not with the poorer class of animal, it is the rule rather than the exception that they may develop an udder and come into milk without ever having been mated. The onset of such a lactation comes commonly in the spring, which is also the end of the breeding season, and the yield, though less than what would be given after kidding, may yet be as great as