Introduction: In the animal kingdom, External fertilization is a common reproductive strategy. The union of the sperm and the egg produced by the male and female organisms occurs outside the female body. Mostly aquatic organisms tend to go through external fertilization, to facilitate the locomotion of the sperms underwater. This process is alternatively also known as ‘Spawning.’ The Embryo generally develops and attains maturity in the external surroundings.
Another standard type is known as Internal fertilization. It is a biological process in which the fusion of the sperm and the egg occurs inside the female organism’s body. For this to happen, the male sperm has to be introduced into the female body’s reproductive tract through an intromittent organ. But, the development and maturity of the Embryo may take place either internally or externally. Based on varied methods of reproduction, organisms can be classified into oviparous and viviparous animals.
Now, we shall discuss oviparous and viviparous animals with examples.
The Latin terminology for viviparous is also known as ‘Viviparus.’ This means, ‘life-bearing’ or ‘to bring forth alive.’ Animals who can give birth to the younger ones are called viviparous animals. In the case of viviparous animals, since they do not hatch eggs, embryonic development takes place inside the mother’s body, i.e., the female reproductive system. Once the fetus is fully developed, the baby is delivered from the mother’s body.
This concept is related to ‘Matrotrophy’ in which the embryo directly attains the additional supply of nutrition from the mother.
Examples: Marine animals such as whales, dolphins, dogs, cats, and human beings. Most of the mammals fall under this type.
In the case of oviparous animals, the primary process of fertilization takes place internally. The mothering parent produces the eggs. The eggs are not retained inside the mother’s body throughout embryonic development. After the fetus obtains full maturity inside, the egg hatches. This is an evolutionary method of reproduction because a large number of eggs can be laid.
The eggshells are responsible for keeping the Embryo protected inside from predators. In oviparous animals, the Embryo's primary source of nutrition lies in the yolk material inside the eggshell, which is solely deposited by the mother’s reproductive system. Such a condition is known as ‘Lecithotrophic,’ unlike matrotrophic.
Examples: Mostly amphibians, reptiles, birds follow such reproductive strategies. Insects, molluscs, arachnids, and monotremes are examples of oviparous animals.
The Process of Metamorphosis in Viviparous Animals
All organisms grow into adults after the young ones are born. This process of growth is subjected to different stages. In most cases, the offspring might look completely different from the adults—for example, frogs, silkworms, butterflies, etc.
For example- From eggs - tadpoles -further to adult frogs.
From caterpillar- pupa- adult silkworms, such organisms go through distinct changes while growing up. This process of transformation of the young ones into adults by undergoing a sequential chain of changes or rather a development is known as ‘Metamorphosis.’
The Process of Ovoviviparity in Ovoviviparous Animals
Ovoviviparity is otherwise also known as, ‘Aplacental viviparity.’
In the case of ovoviviparous animals, the fertilization of the eggs occurs internally due to mating between the male and the female sexual organs. When the eggs are hatched inside the mother’s body, they still tend to remain in the oviducts for a certain period until they are fully ready to be laid outside, matured, and developed to survive in the external environment.
Since the ovoviviparous animals do not have any umbilical cord attachment or any placental appendage with the mother for nutritional needs or gas exchange hence, the primary source of food or oxygen for the growing offspring lies in the yolk content of the egg sacs. This is alternatively also known as ‘yolk-sac viviparity,’ which means that the baby grows without any apparent maternal care.
There are some species like sharks and rays which share a specific outlet for gas exchange with the developing babies in the womb itself. In the case of particular species, due to reduced nutrition levels in the egg yolk, it is often replaced with uterine secretions, such as trophic eggs in the uterus.
Generally, in the case of ovoviviparous animals, by delaying the process of giving birth to the newborns, they become more eligible to defend themselves against the adversities in the wild. They can fulfill their needs without a mother’s protection. This proves to be a significant benefit for the ovoviviparous animals.
As discussed earlier, fertilization alternatively, also known as ‘Syngamy,’ is the fusion of the male and female gametes to reproduce a newborn. In the case of animals, including human beings, the process involves the union of a sperm and an ovum, which eventually leads to the formation of a unicellular zygote, a single diploid cell.
The zygote undergoes a series of mitotic cell divisions to a complex system of tissues and cell types, i.e., the Embryo. This further moves and implants itself to the lining of the uterine walls. This procedure is called ‘Implantation.’ The implanted Embryo eventually grows into the brain, heart, eyes, lungs, and other organs. A wholly developed embryo grows to become a fetus and then, a multicellular organism. This entire process happens over 8-9 months.
After the development of a full-grown fetus in the womb, the baby is delivered.
This discussion on Embryo development or ‘Embryogenesis’ is subjected to vertebrates and mammals' common features, whether fertilization takes place internally or externally.
1. Who are Known as Oviparous Animals?
Ans: These animals can undergo both internal or external fertilization. But the eggs are generally hatched outside because the zygote development takes place outside the female body. Birds, frogs, and other reptiles are significant examples.
2. Are Human Beings Viviparous?
Ans: Yes, they are. Like most mammals, they undergo internal fertilization to give birth to newborns. The zygote formation or the fertilization of the egg cell takes place inside the mother’s body. They are born, once the fetus matures fully over 8-9 months.
3. Which are the Animals that can be Called Ovoviviparous Animals.
Ans: A few examples include platypus, guppies, slow worms, lizards, etc.
4. How are Oviparous Animals Different from Viviparous?
Ans: Oviparous animals can hatch fertilized or unfertilized eggs. The embryogenesis also takes place outside the female body. They generally go through either external or internal fertilization whereas; viviparous animals go through only internal fertilization. They directly give birth to newborns, and the development of the embryo takes place inside the mother’s womb.
5. Which Group of Animals have more Chances of Survival?
Ans: Viviparous animals have more chances of survival because of assured protection and nourishment procured from their mothers.