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Reproduction in Mammals
Most mammals are viviparous, giving birth to their offspring alive. (The only exception to this are the primitive egg-laying mammals of Australia and New Guinea, such as the duck-billed platypus.)
The offspring of mammals are not born immediately after the parents have mated. In mammals a period of development is necessary. This period is called gestation (in humans we call it pregnancy). Gestation takes place in a special organ in the female called the uterus or womb. The uterus is found inside the abdomen close to the ovaries where the eggs are produced.
The sperms, which are placed inside the female during copulation with the male, have to swim through the uterus and up the oviduct towards the ovary. The egg cell which is released by the ovary moves down the oviduct towards the uterus. The egg and sperms meet in the oviduct and it is there that fertilization takes place.
The fertilized egg cell continues its journey floating down the oviduct. At the same time the egg cell will begin to divide.
First it divides into two cells, then into four cells, then eight cells and so on. By the time it arrives at the uterus the fertilized egg cell has become a ball of cells called the embryo. In humans this journey takes 10 to 14 days.
At this point something special happens. The embryo floats into the uterus and sticks to its lining. Then it sinks into the wall of the uterus and disappears from view.
This stage is called implantation and it marks the beginning of gestation. It must be understood that the embryo is no bigger than a pin head at this stage.
Gestation can be divided into three parts. The first part is the development of the embryo. When the embryo is completed we can see a whole baby mammal in miniature. The second part of gestation is the development of the foetus. This is a period of growth; the tiny embryo increases in size. The last part of gestation is called birth when the foetus passes out of the female's body.
The period of gestation depends upon the species of mammal. In general, the bigger the mammal is the longer the gestation period is. You can see this in the table below
This gestation period will also change with the climate and the availability of food for the mother.
Not all gestations are the same. The usual pattern is that the male and female mate. The egg is fertilized by the sperms in the oviduct. The fertilized egg travels to the uterus where the embryo completes its development. In some mammals, for example the stoat, the fertilized egg does not develop immediately. These mammals mate in the summer. If the offspring were to be born in the autumn they would die in the cold winter season.
The fertilized egg of the stoat travels to the uterus where it rests. The embryo does not develop until the spring when the days get longer and the weather is warmer.