Difference Between Internal And External Fertilization

What Is Internal Fertilization?

Internal fertilization is the union of an egg cell with a sperm during sexual reproduction inside the female body. It occurs mostly in terrestrial animals. For internal fertilization to happen there is need for a method of introduction of male sperms into the female reproductive tract.  Different animals have different methods of transferring sperms into their female counter parts’ reproductive tract. Some of the ways include:

• Copulation: This is common in mammals, reptiles, some birds, some fish and certain groups of animals. In this method, a penis or an intromittent organ is introduced into the vagina or cloaca.
• Cloacal kiss: This is common in birds, whereby two animals press their cloacas together while transferring sperm.
• Spermatophore:  salamanders, spiders, some insects and some molluscs undertake internal fertilization by transferring a spermatophore, a bundle of sperm from male to the female.

There are three ways in which internal fertilization produces offsprings:

• Oviparity: In this mode of internal fertilization, fertilized eggs are laid outside the body of the mother. The egg receives nourishment from the York. For example fish, amphibians, reptiles and all birds.
• Viviparity: In this mode of internal fertilization, the young ones develop inside the mother and receive nourishment through the placenta.  
• Ovoviviparity:  In this mode of internal fertilization, eggs are retained in female and the embryo receives nourishment from the York. When they hatch, the young ones are fully developed. This is common in some bony fish, sharks, lizards, snakes etc.

What You Need To know About Internal Fertilization

1. Internal fertilization refers to the fusion of gametes inside the female organism during sexual reproduction.
2. Three modes of internal fertilization are oviparity, viviparity and ovoviviparity.
3. Internal fertilization occurs in mammals, reptiles, some birds and some fish.
4. Further process of development (syngamy) occurs inside the body only.
5. In plants, internal fertilization occurs in bryophytes, pteridophytes, gymnosperms and angiosperms.
6. Internal fertilization shows high survival rates of the embryo and egg.
7. Internal fertilization can be successful in both dry and moist environments.
8. Less number of gametes are produced during the internal fertilization.
9. Internal fertilization does not necessarily result in greater genetic diversity.
10. The number of offspring that can be produced through internal fertilization are few in number.

Advantages Of Internal Fertilization

• The fertilized egg is protected from predators and harsh climatic changes in the environment.
• There is lesser chance of desiccation of gametes.
• The chances of a successful fertilization are high.
• The mates are selective.
• Chances of survival of the offspring are high.

Disadvantages of Internal Fertilization

• There is a lesser number of offspring produced at a given time.
• It is sometimes difficult for the male and female to come into intimate contact.
• Males must produce a large number of sperms.
• It requires close coordination between males and females in terms of behavior and physiology which requires extensive hormonal controls.

What Is External Fertilization?

External fertilization refers to a mode of reproduction whereby a male organism’s sperm fertilizes a female organism’s egg outside of the female’s body.  External fertilization typically occurs in water or a moist area to facilitate the movement of sperm to the egg. The release of eggs and sperms into the water is referred to as spawning. In water, the male and female must both release gametes at same time in order to fertilization to take place.

In vertebrates, external fertilization is most common in amphibians and fish. Invertebrates that commonly use external fertilization are mostly benthic, sessile or both, including animals such as coral, sea anemones and tube-dweling polychaetes.

What You Need To Know About External Fertilization

1. External fertilization refers to the fusion of gametes outside the female organism during sexual reproduction.
2. External fertilization occurs in the external environment.
3. External fertilization occurs in frog, fish, mollusks and crustaceans.
4. Further process of development (syngamy) occurs outside the body.
5. External fertilization occurs in most algae.
6. External fertilization shows lower survival rates of the embryo and egg.
7. External fertilization is highly successful in water or moist environments.
8. A large number of gametes are produced during the external fertilization.
9. External fertilization results in a greater genetic diversity.
10. External fertilization produces a large number of offspring.  

Advantages Of External Fertilization

• Large number of offspring is produced.
• It is easier to find mates as the gametes just need to be released and can drift away with wind or water.
• External fertilization results in greater genetic variation.

Disadvantages Of External Fertilization

• Large number of unfertilized gametes will be wasted and there is higher rate of desiccation of the gametes.
• Predators and other environmental hazards reduce the probability of fertilization.
• The sperm might not necessarily fuse with the egg.
• Males must produce large number of sperms to ensure that they travel throughout the water body to meet an egg.
• Females must deposit dozens or hundreds of eggs for successful fertilization.

Difference Between Internal And External Fertilization In Tabular Form

Similarities Between Internal And External Fertilization

• Both occurs in plants and animals
• The final product of both internal and external fertilization is the zygote.
• Both processes involve fusing male gametes with female gametes.
• Mobile male games and immobile female gametes are produced in both internal and external fertilization.