what is external fertilisation
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http://en.wikipedia.org/wiki/External_fertilization
External fertilization is a reproductive process in which fertilization, the union of sperm and egg cells, occurs outside the body of the organism. This process is typically found in aquatic or moist environments and is common among many species of fish, amphibians, and invertebrates.
To understand how external fertilization works, let's break it down into steps:
Step 1: Gamete Release
In external fertilization, both males and females release their gametes (sperm and eggs) into the surrounding environment. This usually happens during reproductive events such as spawning or mating rituals.
Step 2: Gamete Encounter
Once the gametes are released into the environment, the sperm and eggs come into contact with each other. Typically, the sperm swim or drift towards the eggs, attracted by chemical signals or environmental cues.
Step 3: Fertilization
When the sperm reaches the egg, it penetrates and fertilizes it, forming a zygote. This union of genetic material from both parents initiates the development of a new individual.
Step 4: Development
After fertilization, the zygote develops into an embryo and eventually into a fully formed offspring. The development may take place externally, within protective structures like eggs or gelatinous masses, or in the case of some aquatic organisms, the zygote might develop directly in the surrounding water.
External fertilization offers certain advantages and challenges. It allows for the production of a large number of offspring since the chances of gamete encounter are higher in open water or moist environments. However, it also exposes the gametes and embryos to external factors such as predation, temperature changes, and water currents, which can impact the survival of the offspring.
In contrast to external fertilization, internal fertilization occurs when the sperm fertilizes the egg within the female's body. This reproductive strategy is seen in mammals, birds, reptiles, and some fish and provides protection and nourishment to the developing embryos.