contains the embryo, to exchange waste and nutrients with the cytoplasm. There is no official term for platypus young, but the term "platypup" sees unofficial use, as does "puggle". Newly hatched platypuses are vulnerable, blind, and hairless, and are fed by the mother's milk. Although possessing mammary glands, the platypus lacks teats. Instead, milk is released through pores in the skin. The milk pools in grooves on her abdomen, allowing the young to lap it up. After they hatch, the offspring are milk-fed for three to four months. During incubation and weaning, the mother initially leaves the burrow only for short periods, to forage. When doing so, she creates a number of thin soil plugs along the length of the burrow, possibly to protect the young from predators; pushing past these on her return forces water from her fur and allows the burrow to remain dry. After about five weeks, the mother begins to spend more time away from her young, and at around four months, the young emerge from the burrow. A platypus is born with teeth, but these drop out at a very early age, leaving the horny plates it uses to grind food. Evolution The platypus and other monotremes were very poorly understood, and some of the 19th century myths that grew up around themfor example, that the monotremes were "inferior" or quasireptilianstill endure. In 1947, William King Gregory theorised that placental mammals and marsupials may have diverged earlier, and a subsequent branching divided the monotremes and marsupials, but later research and fossil discoveries have suggested this is incorrect. In fact, modern monotremes are the survivors of an early branching of the mammal tree, and a later branching is thought to have led to the marsupial and placental groups. Molecular clock and fossil dating suggest platypuses split from echidnas around 19–48million years ago. The oldest discovered fossil of the modern platypus dates back to about 100,000 years ago, during the Quaternary period. The extinct monotremes Teinolophos and Steropodon were once thought to be closely related to the modern platypus, but are now considered more basal taxa. The fossilised Steropodon was discovered in New South Wales and is composed of an opalised lower jawbone with three molar teeth (whereas the adult contemporary platypus is toothless). The molar teeth were initially thought to be tribosphenic, which would have supported a variation of Gregory's theory, but later research has suggested,