External vs Internal Fertilization
Fertilization is one step of the reproduction process both in animals and plants. Reproduction can be categorized as sexual reproduction and asexual reproduction, and fertilization only occurs in sexual reproduction. The term fertilization mostly refers to production of a new organism through the combining of ovum and sperm. In fertilization, haploid ovum combines with the haploid sperm to form a diploid zygote making genetic mixture.
There are two types of gametes; male and female. In flowering plants, since the swimming of male gamete is absent due to the adaptation to the land, pollination allows fertilization. As a result of the germination of pollen grain (male gamete) in response to the sugary fluid of the stigma, pollen tube grows and can reach the female gamete. The pollen tube enters to the ovule through the micropyle. Pollen tube reaches near the bottom of the ovary and, near the receptacle, it breaks into the embryo sac and eventually zygote becomes the fruit.
In lower plants, male gamete is motile, and swims to the immobile female gamete and fertilization occurs. Zygote becomes the sporophyte.
The fertilization process occurs within the body of a female is called internal fertilization. It is a specialization for the protection of egg, but it depends on the birth method . Reptiles and birds have thick shell, covering the egg to protect it from dehydration and destruction. However, fertilization occurs inside the body otherwise, sperm has to enter through a thick wall.
Animals such as mammals also have internal fertilization, where embryo develops inside mother, which enhances the protection to the embryo1.
Internal fertilization facilitates the survival of the embryo, selective fertilization, and longer protection and minimizes the wastage of gametes.
In external fertilization fusion of sperm and egg occurs externally of the female body. External fertilization needs water to facilitate their fertilization, so it occurs in wet environments. Female and male gametes are released to the water, and male gamete is mostly mobile. This type of fertilization can be seen in lower plants. The advantage of external fertilization is that it produces a large number of offspring due to the external hazards. So survival of the embryo is comparatively lower. Amphibians and fish are examples for these types of animals.
What is the difference between Internal fertilization and External fertilization?
• The internal fertilization process occurs within the body of female whereas, in external fertilization, fusion of sperm and egg occurs externally of the female body.
• After the internal fertilization, egg will come out of the body having a thick shell whereas, in external fertilization, eggs are produced with thin tertiary membrane or without membrane .
• External fertilization needs water, whereas internal fertilization does not need water to fertilize.
• Organisms involved in external fertilization have mobile male gametes with flagella, whereas organisms involved in internal fertilization has immobile male gametes.
• In internal fertilization, wastage of gametes is lower, whereas wastage of gametes is higher in external fertilization.
• Organisms that involved in internal fertilization produce lower number of gametes, whereas organisms involved in external fertilization produce a large number of gametes.
• Survival of organisms that involved in internal fertilization is higher than the survival of organisms involved in external fertilization.