INSIDE THE WOMB
Take a look inside the mammalian womb and discover how unborn cubs, kittens and calves grow
Within the animal kingdom, there are three types of mammals: placentals, marsupials and monotremes. Placentals, like humans, carry a foetus within their uterus, also known as the womb. Once a fertilised egg attaches to the lining of the uterus, it develops over several weeks into an early embryo. Along with the development of an embryo, a new temporary organ called the placenta grows.
During development, the embryo produces root-like structures called villi, which anchor it in the womb. These villi grow, eventually connecting to the blood vessels of the mother, and the placenta starts to grow. The purpose of the placenta is to deliver nutrients and oxygen to the foetus. The connection between the foetus and the placenta is called the umbilical cord, which leaves a belly button after birth.
To keep the foetus safe during gestation, an encompassing sac of fluid called an amniotic sac also develops to cushion it before it’s ready to be born. A placental pregnancy can last anywhere from 11 days to almost two years, depending on the species. But mammals aren’t alone in having placental young. Many species of fish, such as sharks, and some reptiles, including skinks, also rely on a placenta to grow their offspring.
Marsupials have a shorter gestation time than placental mammals, but provide their young with an extended period of lactation. Once a marsupial foetus has spent anywhere from 8 to 42 days in the womb, they make their way into a marsupium, an internal pouch that protects the offspring, and latch onto an available nipple for a lactation period that can last up to 300 days. Monotremes are a class of their own. Only five species are included in this strange group of mammals, including platypus and echidnas. Ditching a womb altogether, these animals pass eggs through an organ called a cloaca.