If the egg is not fertilised, it lives for about one day. Since the ovary releases one egg every month, the uterus also prepares itself every month to receive a fertilised egg. Thus its lining becomes thick and spongy. This would be required for nourishing the embryo if fertilisation had taken place. Now, however, this lining is not needed any longer. So, the lining slowly breaks and comes out through the vagina as blood and mucous. This cycle takes place roughly every month and is known as menstruation. It usually lasts for about two to eight days.
The reproductive parts of angiosperms are located in the flower. The different parts of a flower are sepals, petals, stamens and carpels. Stamens and carpels are the reproductive parts of a flower, which contain the germ cells. The flower may be unisexual (papaya, watermelon) when it contains either stamens or carpels or bisexual (Hibiscus, mustard) when it contains both stamens and carpels. Stamen is the male reproductive part and it produces pollen grains that are yellowish in colour. You must have seen this yellowish powder that often sticks to our hands if we touch the stamen of a flower. Carpel is present in the centre of a flower and is the female reproductive part.
There are different methods of contraception to avoid pregnancy. These contraceptive methods fall into a number of categories.
The first one in this category is the creation of a mechanical barrier so that sperm does not reach the egg. 1. It can be by use of condoms. 2. Usage of vaginal cap.
The second type of contraceptive is by changing the hormonal balance of the body so that eggs are not released and fertilisation cannot occur. The drugs used for this purpose is commonly taken orally as pills. However, since they change hormonal balances, they can cause side effects too.
The third type of contraceptive is the usage of devices such as the loop or the copper-T that is placed in the uterus to prevent pregnancy. Again, they can cause side effects due to irritation of the uterus.
Permanent methods include surgical methods. 1. Vasectomy in the vas deferens of the male blocks sperm transfer. 2. Tubectomy in the fallopian tube of the female blocks, the egg from reaching the uterus. In both cases fertilisation will not take place.
While surgical methods are safe in the long run, surgery itself can cause infections and other problems if not performed properly.
Surgery can also be used for removal of unwanted pregnancies. These may be misused by people who do not want a particular child, as happens in illegal sex-selective abortion of female foetuses.
For unicellular organisms, cell division, or fission, leads to the creation of new individuals. Many different patterns of fission have been observed. Many bacteria and protozoa simply split into two equal halves during cell division. In organisms such as Amoeba, the splitting of the two cells during division can take place in any lane. However, some unicellular organisms show somewhat more organisation of their bodies, such as is seen in Leishmania (which cause kala-azar), which have a whip-like structure at one end of the cell. In such organisms, binary fission occurs in a definite orientation in relation to these structures. Other single-celled organisms, such as the malarial parasite, Plasmodium, divide into many daughter cells simultaneously by multiple fission.
In multi-cellular organisms with relatively simple body organization, simple reproductive methods can still work. Spirogyra, for example, simply breaks up into smaller pieces upon maturation. These pieces or fragments grow into new individuals. This is not true for all multi-cellular organisms. They cannot simply divide cell-by-cell. The reason is that many multi-cellular organisms, as we have seen, are not simply a random collection of cells. Specialised cells are organised as tissues, and tissues are organised into organs, which then have to be placed at definite positions in the body. In such a carefully organised situation, cell-by-cell division would be impractical. Multi-cellular organisms, therefore, need to use more complex ways of reproduction.
A basic strategy used in multi-cellular organisms is that different cell types perform different specialised functions. Following this general pattern, reproduction in such organisms is also the function of a specific cell type. There must be a single cell type in the organism that is capable of growing, proliferating and making other cell types under the right circumstances.
Every species has to constantly struggle for its survival. Natural predators and vagaries of nature keep on removing a large section of the population of a particular species. Moreover, the natural cycle of life and death also removes a section of the population. Reproduction is a way to replenish the lost section of population. Thus, it can be said that reproduction is linked to the stability of population of a species.
Pregnancy will make major demands on the body and the mind of the woman, and if she is not ready for it, her health will be adversely affected. Therefore, many ways have been devised to avoid pregnancy. These contraceptive methods fall in a number of categories. One category is the creation of a mechanical barrier so that sperm does not reach the egg. Condoms on the penis or similar coverings worn in the vagina can serve this purpose. Another category of contraceptives acts by changing the hormonal balance of the body so that eggs are not released and fertilisation cannot occur. These drugs commonly need to be taken orally as pills. However, since they change hormonal balances, they can cause side effects too. Other contraceptive devices such as the loop or the copper-T are placed in the uterus to prevent pregnancy. Again, they can cause side effects due to irritation of the uterus. If the vas deferens in the male is blocked, sperm transfer will be prevented. If the fallopian tube in the female is blocked, the egg will not be able to reach the uterus. In both cases fertilisation will not take place. Surgical methods can be used to create such blocks. While surgical methods are safe in the long run, surgery itself can cause infections and other problems if not performed properly.