In some other cases, the eggs may result from mitotic division, where they develop to diploid (2N) individuals from the beginning as in case of the Aphid insect.

Parthenogenesis has been induced artificially by egg activation as in case of frogs and sea stars through it exposure to heat, electric shock, irradiation, some salts, agitation, or pricking with a needle. These stimuli lead to duplication of their chromosomes and so develop without fertilization to individual that are totally identical with the mother. By similar stimuli, early embryonic stages of rabbits were obtained from their eggs.

6. Tissue culture:

Scientists carried out experiments of tissue culture on Plants and Animals. They grow their tissues in a semi-natural nutrient medium and follow their differentiation and progress till full organisms are obtained. In an exciting experiment a scientist separated small pieces of Carrot plant into conical flasks containing Coconut milk, which comprises the whole plant hormones and nutrient elements. They began to grow and develop into a full Carrot plant. He separated single cells from the same plant and cultivated them by the same method and obtained full plants. Similarly, Tobacco plants were obtained from isolated cells of Tobacco leaves that have been treated by the same method. These experiments confirmed that any somatic cell in the plant comprises the whole genetic information that can be translated a whole developed organism if cultivated in a proper nutrient medium containing plant hormones with certain ratio. These methods are now applied in propagating rare plants or desirable strains or more resistant to diseases. Selected tissues for such culture can be preserved in liquid Nitrogen for a long period while remaining alive till cultivation. Hence, scientists foresee hopes on the progress of these techniques nearly in order to solve food problems, and to shorten the time needed for growing desirable crops.

Second: Sexual Reproduction:

Sexual reproduction necessitates the presence of 2 individuals, a male and a female for the production of sexual gametes. Gametes should meet for fusion during fertilization. At mating male gamete meets with the proper female gamete and fuse to form the zygote. Zygote then divides and grows to the embryo, then to the young and the adult that comprises parents’ features. So, the young receives from both parents even a minute part of their nuclear substance, and thus becomes a blend from both. Conversely, the young in the Asexual reproduction receives nuclear substance from only one parent and so becomes a copy from it. However, sexual reproduction is more time, and energy consuming than the Asexual reproduction, since it occurs after elapse of certain age and preparation. In some cases, the parent should prepare the proper nest or burrow before mating. They may alternate in guarding the eggs, and protection of the young till they grow larger. Some forms even face more hardships in order to protect their young, as they keep their embryos within their bodies till birth. These young may remain with their parents in a social life for more protection and learning more about their behaviour. Beside, production of new individuals is limited here to half the number of organisms, females only but not the males. In Asexual reproduction, all the individuals multiply. In spite of all the previous, Sexual reproduction provide the descendants with continuous innovations in their genetic contents that enable them to continue in the face of environmental variation.

The sexual reproduction depends on meiotic division, when forming gametes, during which the number of chromosomes is reduced to its half, so cells resulting from this division are haploid (N). During fertilization, the male gamete (N) fuses with the female gamete (N) to restore the original chromosomal number (2N), which differs according to the species.

Types of sexual reproduction:

1. Conjugation:

It takes place in some primitive organisms such as some Protozoa, Algae, and Fungi where reproduction takes place by mitosis at suitable conditions, but they turn to reproduce sexually by conjugation if subjected to drought, or a change in the water temperature or purity.

Conjugation in Spirogyra:

Spirogyra is common in the green scum of standing water, where its filaments float. Each filament is formed of one row of cells. This Alga turns to conjugate on advent of unsuitable conditions. Conjugations in Spirogyra are of two types:

A. Scalariform conjugation in Spirogyra:

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