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Reproduction in Mammals : During Gestation
Once the embryo has been implanted in the wall of the uterus it begins to grow. (The embryo of a bird has a sack of yolk to feed upon placenta but the embryo of the mammal is fed directly by its mother.)
As the embryo grows it remains attached to the wall of the uterus by a connection called the umbilical cord. At the end of the umbilical cord, attached to the wall of the uterus, is a special organ called the placenta. The placenta takes all the food and oxygen that the embryo needs from the mother's blood. At the same time it gives the mother's blood all the wastes, such as carbon dioxide, which the embryo produces. The mother's body will remove these wastes because some of the organs in the embryo, such as the lungs, are not working yet.
The food and oxygen taken from the mother by the placenta pass down the umbilical cord in the embryo's own blood stream.
The embryo is surrounded by a membrane called the amnion which contains the amniotic fluid. This membrane and the amniotic fluid protect the embryo as it grows and develops.
At the end of pregnancy the offspring are born one at a time. First the amnion breaks and the amniotic fluid is lost through the mother's vagina. Then the muscles in the wall of the uterus push the foetus out of the uterus. The contractions of the muscles may begin many hours before the birth of the offspring. These contractions become stronger and more regular as the birth of the offspring approaches. The foetus usually passes head-first through the vagina and out of the mother's body - this normally only takes a few minutes.
When the baby is born it still has its umbilical cord attached to the placenta inside the uterus. Soon after the baby is born the placenta pulls away from the uterus and follows the baby with the rest of the umbilical cord, out of the mother's body.
The baby will start to breathe on its own straight away so the placenta is no longer needed. Amongst wild mammals the first thing that the mother will do is to clean the babies by licking them dry. She will also cut the umbilical cord by biting through it near the baby's body. No blood will be lost because the umbilical cord does not function any more. Finally, the mothers of many species of wild mammals often clear away the amnion, placenta and umbilical cord by eating them. This is important because the blood in the placenta could attract predators to the helpless, new-born offspring.
Humans are special because the mother is helped during childbirth by a nurse or a doctor. When the baby human is born the nurse will clean the baby and cut the umbilical cord. A thread is tied round the end of the umbilical cord before it is cut to stop any bleeding. A dressing is put over the cut end of the cord to prevent any infection. Finally the rest of the cord and the placenta are disposed of in the hospital.
In cleaning her offspring for the first time the mother also gets to know them by their smell. This is very important for mammals which live in herds, such as sheep or horses. The mother and her offspring often identify one another by their smell.
There are only three species of egg-laying mammals: the duck-billed platypus and two species of spiny anteaters or echidnas. Egg-laying mammals are only found in Australia and New Guinea. They are all quite rare. The eggs of these animals are fertilized inside the female. The fertilized egg is covered in layers of albumen and a soft shell as it passes down the oviduct. It takes between 12 and 20 days to lay the egg (in birds this only takes 24 hours). The female duck-billed platypus lays her eggs at the end of a tunnel, but the spiny anteaters keep their eggs in a pouch. The eggs hatch after about 10 days and the offspring feed on their mother's milk for 3 to 4 months.
These mammals are also found mainly in Australia and New Guinea. A number of marsupials, however, can also be found in North and South America.
The word "marsupial" comes from the Latin word marsupium which means "little bag". The embryos of marsupials only spend a short time inside the uterus of their mother (usually between 12 and 28 days). When they are born marsupial offspring are very small. The grey kangaroo is only 0,8 g at birth but it can grow to over 30 kg as an adult.
To complete its growth the baby marsupial will have to leave its mother's uterus, crawl up the mother's belly and into her pouch. When it climbs inside the pouch the baby will find a nipple which will supply it with milk. The baby grey kangaroo spends 300 days in its mother's pouch. At the end of this time it weighs 5 kg. The young kangaroo continues to feed on its mother's milk until it is about 18 months old.