Viviparous and Oviparous Embryo Development

Oviparous and Viviparous are two categories of animals divided based on embryo development. Oviparous animals reproduce by laying eggs that mature and hatch in the external environment and Viviparous animals hold fertilized eggs inside their bodies and give birth to their young. These reproductive modes are known respectively as "oviparity" (egg-laying reproduction) and "viviparity" (live-bearing reproduction). The development of an embryo may take place internally in viviparous animals or externally in oviparous animals but the stages of development remain the same.

👁 Viviparous-and-Oviparous-Animals

Modes of Reproduction

Living beings reproduce by either asexual or sexual means. Offspring produced through asexual reproduction share the same genetic makeup as their single parent because they are all clones of the original parent. In sexual reproduction, the genetic material of both parents is contributed to create a unique offspring. Two types of reproduction modes seen those afre:

• Ovivarous
• Viviparous
• Ovoviviparity

Oviparity (Egg-laying)

Oviparity is a reproductive mode in which females deposit eggs that mature and hatch in the external environment. The oviparous animals can perform internal as well as external fertilization. Depending on the kind of fertilization, the females lay fertilized or unfertilized eggs. In certain oviparous animals, the females retain their fertilized eggs for certain period of time before laying them; allowing the embryos to be in an advanced stage of development upon oviposition. This feature is known as "oviparous egg retention," where the female normally lays an intact egg which continues to develop in the external environment before it hatches.

Examples of Oviparous Animals

• Birds: All the birds lay eggs and sit on them in order to keep them warm and take care until it hatches to give birth to the young one.
• Amphibians and Reptiles: All the fishes and amphibians lay eggs. All other reptiles are oviparous, with the exception of a few species of snakes.
• Fishes: Although they also lay eggs, but their eggs are in an unfertilized state since the fishes undergo external fertilization.
• Mammals: Examples of mammals that lay eggs are echidna and platypus.

Viviparity (Live-bearing)

In contrast, viviparity is a reproductive pattern in which females give birth to offspring that are able to live freely. They do so by retaining developing eggs inside their body. Despite interspecific variations in placental complexity, they possess the capability to maintain eggs in the uterus for the entire process of embryo development, resulting in the production of fully developed offspring at birth. During gestation, the mother gives nutrition to the embryo by the phenomenon known as "matrotrophy."

Examples of Viviparous Animals

1. Marsupial Viviparous animals: They lack the placental system. After the birth of an immature foetus, the mother keeps the baby in specialized pouches for the purpose of protection and feeding. Example: Kangaroos.
2. Placental Viviparous animals: They bears a specialized system that keeps the check on the requirement and protection of the baby inside the mother’s womb. The placenta remains attached to the lining of mother’s uterus and provides the nutrients through the bloodstream. Example: Humans.
3. Ovoviviparity: It is the blend of oviparity and viviparity where the female bears the eggs internally. And once after the maturation of the baby is complete, the young one hatches inside the mother’s womb and then it is delivered outside. Examples: Sea horses, ray fish, etc.

Embryonic Nutritional Patterns

Animlas can also be categorised based on the way in which the embryos receive nourishment. In most animals, the yolk of the egg contains nutrients necessary for growth. "Lecithotrophy" is the name used to describe this feeding pattern in embryos: The Greek term lecithos means "yolk." This pattern differs from "Matrotrophy," when the female supplies nutrition for the development of the foetus through a placenta ("placentotrophy") or some similar structure.

Comparison between Oviparous and Viviparous animals

Stages of Embryo Development

Emrypo development stages are given below:

1. Development of embryo begins with the fertilization of an ovum (egg cell) by a sperm, resulting in the formation of a single diploid cell known as a zygote.
2. The zygote undergoes series of mitotic divisions without any significant growth (a process termed as cleavage) to form a structure termed as morula.
3. By a process called blastulation, the morula develops into a structure called as blastula, which eventually develops into a structure called as gastrula by a process called gastrulation, when pluripotent stem cells differentiate into the three germ layers:ectoderm, mesoderm and endoderm. The ectoderm gives rise to the skin and the nervous system. The mesoderm is responsible for the development of numerous cell types like muscle, bone, and connective tissue. Endoderm layer become the linings of the digestive and respiratory system, and form organs like liver or pancreas.
4. The gastrula further undergoes development which includes organogenesis i.e., formation of organs.
5. Finally, the embryo migrates to the uterus and attaches to the wall of uterus. This phenomenon is called implantation. In due course of time, the implanted embryo develops different body organs such as the eyes, hands, heart, lungs,legs, etc.
6. A completely developed embryo is called a foetus. This entire period is known as pregnancy. Once the foetus is fully mature, the mother delivers the baby.

Metamorphosis in Oviparous Animals

👁 Metamorphosis

The term “metamorphosis” is a Greek word meaning to transform. It is a process by which animals undergo rapid and extreme physical changes after birth. The outcome of metamorphosis may be change to entire body plan of the organism eg. change in the number of legs, its mode of eating or breathing. It is majorly required for sexual maturity. The changes leading to metamorphosis are triggered by hormones.

Examples include the process undergone by most insects such as butterflies, silkworms, cockroach, etc. Amphibians are the only animals with backbones that can undergo metamorphosis. For eg., there are three stages in frogs and their appearance at each stage varies. They begin as an egg, then become a larva (tadpole) and finally transform into an adult frogs.

Conclusion

After going through the above article, we can conclude that both the reproductive strategies namely Oviparity and Viviparity are equally important and play a very dynamic role in reproduction. Also, we can say that the ultimate aim of each and every organism is to increase the number of its species. We also came to know the basic differences between oviparous and viviparous animals and their distinct reproductive strategies.