milk glands, which are absolutely peculiar to the Mammalia, are still in a very primitive stage, and do not yet produce milk proper; and there is only a temporary shallow marsupium.
19. Stage of Metatheria, or Marsupialia, are direct descendants of Prototheria; but they show higher development by the reduction of the coracoid bones and the inter-clavicle. The original cloaca is divided into a rectal chamber and a uro-genital sinus, completely separated, at least in the males; they are viviparous; the young are received into a permanent marsupium, in the walls of which are formed typical milk glands and nipples, but the embryo is still devoid of a placenta, although some recent marsupials show indications of such an organ. The corpus callosum in the brain is still very weak.
Most of the marsupials are extinct. They occur from the Upper Trias onwards, and had in the Jurassic epoch attained a wide distribution both in Europe and in America.
