Introduction
When male and female gametes are fused, zygote formation takes place. Zygote is considered as a vital link that ensures the continuity of species generation after generation. Every sexually reproducing organisms including human beings start life as a single cell, called the zygote. After zygote formation, process of development of embryo takes place. During embryogenesis, zygote undergoes mitotic cell division and cell differentiation. On the basis of development of zygote, animals are classified into different groups such as oviparous, viviparous and ovoviviparous. The following article discusses the meaning of oviparous, viviparous and ovoviviparous in brief.
Oviparous animals
They are egg producing animals in which eggs mature and hatch outside the parent body. Eggs can either be fertilized or unfertilized before they are expelled from the parent body. For example, frog is an oviparous animal, but external fertilization occurs in frogs. On the other hand, birds and reptiles are also oviparous, but they possess internal fertilization. So, we can say that when an animal lays egg – it could either be fertilized egg or unfertilized egg. For instance, eggs laid by chickens are not always fertilized eggs, they could also be unfertilized depending on whether the female hen has undergone mating with the male. So, when a chicken lays eggs without mating – eggs would be unfertilized. And if a female chicken undergoes mating before hatching – eggs would be fertilized. The development of zygote occurs outside the body of the female.Embryo receives nutrition from the yolk. However, the chances of survival are very less because they are exposed to the external environment. Examples: birds, reptiles, amphibians, most fishes, insects, etc. A few exceptions in mammals showing oviparous characters are – duck-billed platypus, echidna or spiny ant eaters, etc.
Viviparous animals
Animals that give birth to live young individuals. In viviparous animals, fertilization takes place inside the female parent body. After fertilization, zygote is formed and then zygote undergoes the process of embryogenesis to form a new individual. In other words, in viviparous animals – fertilization, zygote formation and embryogenesis occur inside the body of the female parent. For example, mammals are viviparous. True viviparity is shown by the formation of chorioallantoic placenta. It is shown in a specific group of mammalia i.e., eutheria – placental mammals. In viviparous animals, embryo receives nutrition directly from the mother through placenta. However, the chances of survival are more as they are protected from the external environment. Example: most mammals
Ovoviviparous animals
These animals show internal fertilization — produce eggs, but instead of laying eggs outside the body, eggs remain inside the mother’s body. Eggs hatch within the mother’s body. However, embryo gets nutrition from yolk stored in the egg. This is also known as ‘yolk-sac viviparity’. In other words, we can say that ovoviviparous animals are like viviparous animals but differ in the absence of placental connection between the mother and the growing embryo.However, if we compare ovoviviparous organisms from oviparous organism – the former is more advanced form as the developing fetus gets mother’s protection from the external environmental adversities. Examples include some sharks (basking shark) and scorpions.