polygynous mating system. Females are thought likely to become sexually mature in their second year, with breeding confirmed still to take place in animals over nine years old. Outside the mating season, the platypus lives in a simple ground burrow, the entrance of which is about above the water level. After mating, the female constructs a deeper, more elaborate burrow up to long and blocked at intervals with plugs (which may act as a safeguard against rising waters or predators, or as a method of regulating humidity and temperature). The male takes no part in caring for his young, and retreats to his year-long burrow. The female softens the ground in the burrow with dead, folded, wet leaves, and she fills the nest at the end of the tunnel with fallen leaves and reeds for bedding material. This material is dragged to the nest by tucking it underneath her curled tail. The female platypus has a pair of ovaries, but only the left one is functional. The platypus's genes are a possible evolutionary link between the mammalian XY and bird/reptile ZW sex-determination systems because one of the platypus's five X chromosomes contains the DMRT1 gene, which birds possess on their Z chromosome. It lays one to three (usually two) small, leathery eggs (similar to those of reptiles), about in diameter and slightly rounder than bird eggs. The eggs develop in utero for about 28 days, with only about 10 days of external incubation (in contrast to a chicken egg, which spends about one day in tract and 21 days externally). After laying her eggs, the female curls around them. The incubation period is divided into three phases. In the first phase, the embryo has no functional organs and relies on the yolk sac for sustenance. The yolk is absorbed by the developing young. During the second phase, the digits develop, and in the last phase, the egg tooth appears. Most mammal zygotes go through holoblastic cleavage, meaning that, following fertilization, the ovum is split due to cell divisions into multiple, divisible daughter cells. This is in comparison to the more ancestral process of meroblastic cleavage, present in monotremes like the platypus and in non-mammals like reptiles and birds. In meroblastic cleavage, the ovum does not split completely. This causes the cells at the edge of the yolk to be cytoplasmically continuous with the egg's cytoplasm. This allows the yolk, which