Fertilization is the mark the onset of sexual reproduction. In the process, the sperm fuse with the egg (ovum) to produce a zygote. This is a crucial stage in which without fertilization sexual reproduction is vain. The outcome of fertilization is the zygote. The whole process may take place either inside or outside the female parent body. Next stage is the development of the embryo. This stage may also take place inside or outside the mother’s body. Let’s go through the development of the embryo, viviparous and oviparous animals and growth of young one to adult.
Development of Embryo
Fertilization results in the formation of unicellular zygote. Zygote starts to divide and multiply and 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.
Viviparous & Oviparous
In the animal kingdom, external fertilization, sperm-egg fusion takes place externally, outside the female body. The embryo develops and matures in the external environment. While in internal fertilization, 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, viviparous and oviparous.
Viviparous Animals: 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 parent. Once the fetus development is complete, the mother delivers the baby. Human beings, dogs, cats, elephants, etc are few examples of viviparous animals.
Oviparous Animals: Animals that lay eggs are called oviparous. In oviparous animals, fertilization takes place internally but embryo development takes place externally. The eggs of hen, ducks, etc carry immature embryo in them. The hard shells of eggs protect them from damage. Once the fetus is matured, the egg hatches.
Young ones to Adult
Viviparous animals give birth to young ones while oviparous animals lay eggs, eventually grow and become adults. But in few organisms, there are sequential stages of being young ones to being an adult. For example, in frogs, there are three stages. Their appearance each stage differs. They begin as an egg, then become a larva (tadpole) and later become an adult frog. This type of growth stages can be observed in many insects like a butterfly, silkworm, cockroach, etc. The drastic changes of a larva into an adult are called metamorphism.
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