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Introduction to Animal Reproduction

♥ Animals reproduce by giving birth to their offspring alive (viviparous animals) or by laying eggs (oviparous animals).

♥ Reproduction requires two sexes, the male and the female, to produce special sex cells. The sex cell of the male is the sperm and the sex cell of the female is the egg.

♥ Sperm cells are microscopic, they have a flagellum which allows them to swim and they are very numerous.

♥ Sperm cells are made by the organ called the testis in the male.

♥ The egg cells are larger than the sperm cells, they do not have a tail and they are not usually produced in large numbers.

♥ The egg cells are made by the organ called the ovary in the female.


Mating and Fertilization

♥ When the male and female come together to reproduce this is called mating.

♥ The egg cell of the female is fertilized by fusing with a sperm cell from the male.

♥ The egg cells and sperm cells may meet outside the body of the female in water - this is called external fertilization, or the sex cells may meet inside the female - this is called internal fertilization.

♥ If two eggs are each fertilized by a separate sperm a fraternal twin will develop.

♥ If two eggs are each fertilized by a separate sperm a fraternal twin will develop.


Reproduction in Birds

♥ The egg cell of a bird consists of the yolk and the white spot on the yolk which contains the nucleus of the egg cell.

♥ The egg cell leaves the ovary and travels down the oviduct where layers of albumen, two membranes and the shell are added.

♥ Fertilization of the egg cell by a sperm cell must take place before the albumen, membranes and the shell itself are added. If fertilization does not take place formation of the egg continues and it is called an unfertilized egg.

♥ The completely formed egg is usually laid in a nest and it is carefully protected by the parents. The eggs must be kept warm, so the parents will sit on the eggs to incubate them.

♥ The embryo develops inside the egg. It feeds on the yolk and the albumen. The embryo obtains oxygen through the shell which is porous.

♥ The chick hatches from the egg using its egg tooth. In some species the chick is very independent and can feed itself immediately. In other species the chicks must be fed by the parents until they are old enough to fly.


Reproduction in Mammals

♥ Most species of mammal are viviparous. The offspring develop inside the mother in a special organ called the uterus.

♥ The period of development inside the mother is called gestation and it is divided into three parts :
(a) The development of the embryo, when the offspring develops its different organs
(b) the growth of the foetus, when the offspring becomes bigger
(c) birth, when the offspring leaves the mother's body.

♥ The length of gestation depends upon the size of the mammal; big mammals have long gestation periods.

♥ During gestation the embryo and foetus are fed directly by the mother through an organ called the placenta. The placenta is attached to the wall of the uterus.

♥ The offspring is attached to the placenta by an umbilical cord and a fluid-filled sack called the amnion grows around the embryo to protect it.

♥ At birth the baby mammal is pushed out of the mother's body and it begins to breathe and feed for itself.

♥ The first food that all new born mammals feed upon is milk. They suck this from their mother's mammary glands until they can feed upon solid foods.


Different Ways of Growing

♥ The offspring of many animals look like their parents. As they develop they grow bigger but they do not change much in shape.

♥ The offspring of insects and amphibians do not look like the adults, they go through a dramatic change in shape. This is called metamorphosis.

♥ In the first part of its life cycle the insect or amphibian is called a larva and in the second part it is called an adult. In insects there may be a resting stage in between called a pupa or chrysalis.

♥ The larva and the adult stages may feed on different foods or live in different places.

♥ Only the adult can reproduce.


Flowers and Reproduction

♥ Flowers are the reproductive organs of plants. Most flowers are hermaphrodite, containing both male and female parts. Some flowers are unisexual.

♥ The male sex cells are contained in the pollen grains produced in the anthers. The female sex cells are the ovules which develop in carpels.

♥ The pollen of some flowers is transferred by animals, often insects, from one flower to another. These flowers are usually brightly coloured, scented and produce a sugary nectar.

♥ The wind can also transfer pollen. Wind-pollinated flowers are inconspicuous. They are not scented and do not make nectar.

♥ To prevent self-fertilization in hermaphrodite flowers either the male parts develop before the female parts or vice versa.

♥ Pollination is just one step towards fertilization. The male sex cell is taken to the female sex cell by a pollen tube which grows through the stigma, style and carpel.

♥ Fertilization happens when the male and female sex cells meet and fuse together.

♥ When all of the ovules have been fertilized, the carpels develop into fruits. Inside the carpels, the ovules are developing into seeds.


Fruits and Seeds

♥ Plants have developed ways of making sure that their seeds are moved away from the parent plant. This is called seed dispersal.

♥ Some types of seed are dispersed by animals. Some fruits get attached to the animal's fur, while others are eaten and the seeds are either rejected or eliminated by the animal.

♥ Other types of plants use the wind to disperse their seeds. A few use water.

♥ A few plants develop fruits which "explode" when they dry out. This throws the seeds away from the parent plant.


Seed Germination

♥ The seed is covered by a protective coat called the testa. The testa has a scar on it and a small hole called the micropyle.

♥ Inside the seed there is an embryo made up of a tiny radicle and a tiny plumule. There are usually two cotyledons which are the food stores.

♥ The conditions for germination are enough water and warmth. Some seeds need a certain number of daylight hours before they will germinate.

♥ As the seed germinates the radicle grows downwards and the plumule grows upwards. The energy for germination comes from the food stores in the seed.

♥ Once the seedling is established, the green leaves start to make food by photosynthesis. The roots take in water and minerals from the soil.


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