According to J. T. Wilson and J. P. Hill, the ovum of the platypus at the time of fertilization is about three millimeters in diameter ; after fertilization it becomes surrounded by an ex ceedingly thin layer of albumen, and outside this by a thin trans parent horny shell. It grows to a length of 16-18mms. before ovi position. The eggs, which resemble those of certain lizards, are usually laid in pairs which stick together. Burrell infers that when the eggs are laid the animal is sitting curled-up and receives them in its forepaws, by which they are placed between the curled-in tail and the abdomen, where they are brooded. For some time after the young are hatched no milk is secreted ; the young when born have a very short bill with "lips." There are no teats but the young lick and suck the milk from the surface of the mammary field. In the platypus there is no pouch but in the echidna a "brood pouch" is developed.
While the platypus and the echidna differ widely in habits, ex ternal appearance and in skeletal and anatomical characters con nected with the difference in "habitus," they have many impor tant features in common. For instance, both possess a peculiar poison-secreting apparatus, in some ways analagous with those of poisonous snakes. The fang, represented by a large horny spur
attached to each ankle of the male, is better developed in the platypus than in the echidna. The sharp tip of the spur is per forated by a canal leading to a duct running up the leg to a large gland beneath the skin on the upper surface of the femur. The albuminous secretion is poisonous but not fatal to other mammals. The spurs are used in fighting between the males during the breed ing season and also serve for clasping the females.
The existing monotremes are definitely excluded from ancestry to the marsupials or placental mammals by the possession of such curious specializations as the poison gland and tarsal spur. In the other direction there is no known group of extinct Mesozoic mam mals from which we may confidently derive the monotremes, Not withstanding the widespread impression that the monotremes have been derived from the extinct Multituberculata, recently discov ered palaeontological evidence weighs heavily against this view. The anatomy of the monotremes reveals many important features of the brain and skeleton in common with the marsupials, to which group indeed they appear to be more nearly related than to the placental mammals.