mammalstheeggs

Mammal eggs refer to the female reproductive gametes in mammals and to the diverse ways mammals reproduce. In all mammals, fertilization is typically internal, and the resulting embryo develops inside the mother's body in therian species (marsupials and placentals) or, in the egg-laying monotremes, begins development inside an egg laid by the mother.

Monotremes, the egg-laying mammals that include the platypus and echidnas, are unique among mammals for laying eggs rather than giving birth to live young. Their eggs are relatively small and have a leathery shell. The eggs are incubated outside the mother’s body for roughly 10 days to two weeks, after which hatchlings emerge and continue development while nursing from the mother’s milk. Monotreme reproduction combines egg-based development with mammalian lactation.

Therian mammals—marsupials and placentals—do not lay eggs in ordinary reproduction. Oocytes are released during ovulation and fertilized internally. The embryo then develops inside the uterus and, in placental species, is nourished via a complex placenta. The early egg in this context is mainly the oocyte, which contains limited yolk compared with birds or reptiles. After fertilization, the zygote undergoes division and implantation, with nutrient transfer mediated by the placenta or, in marsupials, by a brief intrauterine period followed by extensive postnatal development.

Oogenesis in mammals involves meiotic division to form a mature oocyte capable of fertilization. Fertilization triggers further cellular divisions and the formation of a zygote, which sets the course for embryonic and later fetal development. The study of mammalian eggs highlights the major divergence between egg-laying monotremes and the live-bearing therian mammals.