In the animal kingdom, external fertilization is a type of fertilization where the sperm-egg fusion takes place externally, outside the female body. The embryo develops and matures in the external environment.
While in internal fertilization, the sperm-egg fusion takes place inside the female body. But the development of embryo may take place either internally or externally. Based on this, animals are classified into two, namely, oviparous and viviparous animals.
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Let us have a detailed look at what are oviparous and viviparous animals, and the process of development of their embryo.
Also Refer: Modes Of Reproduction
Animals that give birth to offspring are called viviparous. In viviparous animals, both fertilization, as well as the development of the embryo, takes place inside the female reproductive system. Once the fetus development is complete, the mother delivers the baby. This condition is referred to as matrotrophy where the embryo obtains the nutrients directly from the mother and not the yolk.
Examples of Viviparous Animals
Human beings, dogs, cats, elephants, etc are few examples of viviparous animals.
Animals that lay eggs are called oviparous. In oviparous animals, fertilization takes place internally but embryo development takes place externally.
The eggs of birds such as hen and duck carry immature embryo in them. The hard shells of eggs protect them from damage. Once the fetus is matured, the egg hatches. The trait of egg-laying animals is known as oviparity.
Examples of Oviparous Animals
All birds lay eggs with a typical hard calcium shell. Frogs are egg-laying amphibians which have soft gelatinous eggs requiring constant hydration. Almost all fishes are oviparous. Except for some species of snakes, all other reptiles are oviparous. In mammals, Echidna and platypus are egg-laying.
Explore more: Reproduction in Animals
Metamorphosis in Viviparous Animals
Viviparous animals give birth to young ones. All organisms mature, grow, and eventually become adults. But the process of “growing up” varies. Insects and most other invertebrates undergo a sequential transformation from young ones to adult. This process of a drastic change of a larva into an adult is called metamorphosis. This type of growth stages can be observed in many insects like butterflies, silkworms, cockroach, etc.
The only animals with backbones that can undergo metamorphosis are amphibians. For example, in frogs, there are three stages. Their appearance at each stage differs. They begin as an egg, then become a larva (tadpole) and later become an adult frog.
Explore more: Metamorphosis – Lifecycle Of Frogs And Insects
Ovoviviparity In Ovoviviparous Animals
Ovoviviparous animals lay eggs and develop the eggs inside the mother’s body. The eggs are hatched inside the mother. Once the egg hatches, it remains inside the mother for a period of time and is nurtured from within but not via a placental appendage. Ovoviviparous animals are born live.
Some of the examples of ovoviviparous animals are sharks, rays, snakes, fishes, and insects. Oviparity is different from ovoviviparity in a way that the eggs in oviparity may or may not undergo internal fertilization but are laid and depend on the yolk sac to get nourished till the time they hatch.
Ovoviviparity shows internal fertilization of eggs typically via copulation. For instance, a male shark penetrates his clasper into the female to release sperms. Fertilization of eggs takes place when they are in the oviducts and sustain to develop here, and are supplied by the egg yolk in their egg. The female counterpart of guppies accumulates extra sperms which they use to fertilize their eggs for a period up to eight months. The younger ones remain in the oviducts when the eggs hatch and last there to grow and develop till they mature to be given birth and sustain life.
These animals show no umbilical cord which is typically their physical attachment to the mother for nutrient requirements and gas exchange. In such cases, nourishment is obtained from the yolk of the egg. When this yolk is depleted, the mother provides additional nutrition in the form of unfertilized eggs and uterine secretions.
One of the advantages ovoviviparous animals is that, after birth, the young are competent enough to feed and defend on their own. This means that they can fend for themselves in the wild and are capable of living without the need for their mother’s protection. For instance, rattlesnakes are ovoviviparous and right after birth, they have fully developed venom glands that are as potent as the adult rattlesnakes.
Also Read: Internal Fertilization
Development of Embryo
As stated before, fertilization results in the formation of unicellular zygote. Zygote starts to divide and multiply and eventually develops into an embryo. Embryo moves to the uterus and attaches to uterus walls.
This is called implantation. Implanted embryo eventually develops different body organs such as the heart, hands, legs, eyes, etc. A completely developed embryo is called a fetus. The whole process takes place during the period of 8-9 months. This period or condition is called pregnancy. Once the fetus is mature, the mother delivers the baby.
This is how an embryo develops in humans and animals but this may take place internally or externally.
Also Read: Embryo Development
Difference Between Oviparous, Viviparous And Ovoviviparous Animals
Following are the major differences between oviparous animals, viviparous animals and ovoviviparous animals:
|Viviparous Animals||Oviparous Animals||Ovoviviparous Animals|
|Reproductive Strategy||No eggs are laid. The young ones are born live.||The eggs are laid outside the mother’s body with minimal embryonic development occurring within the mother.||The eggs hatch inside the mother’s body and are nourished by yolk and other fluids until the animal is born.|
|Examples||Almost all mammals.||Birds and Reptiles.||Some species of snakes and sharks.|
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