Question

How many types of fertilization are there? What are they?

Answer 1

Fertilization is the fusion of haploid gametes, egg, and sperm, to form the diploid zygote. The most common series starts with ejaculation during copulation, followed by ovulation, and ends with fertilization.

Complete answer:
There are two types of fertilization namely:
External fertilization :
- This usually occurs in an aquatic environment.
- This kind of fertilization happens during spawning a process wherein one or many females release their eggs and the males release sperm in the same area, at the same time.
- The discharge of the reproductive material may be activated by water temperature or the length of daylight.
- External fertilization in an aquatic environment insulates the eggs from drying up.
Internal fertilization :
- This occurs mostly in land-based animals, although some aquatic animals also come under this category.
There are three sub-categories under internal fertilization:
- In oviparity the fertilized eggs are laid outside the female’s body and develop by obtaining nourishment from the yolk that is a part of the egg. This type of fertilization is seen in bony fish, reptiles, some cartilaginous fish, most amphibians, two mammals, and all birds. Leathery eggs are produced by many reptiles and insects, while birds and turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard.
- Eggs are retained in the female body in viviparity, but the embryo obtains its nourishment from the egg’s yolk and the young are fully developed when they are hatched. This occurs in some bony fish, sharks, lizards, snakes, and some invertebrate animals.
- In viviparity, young ones develop within the female body and derive nourishment from the mother’s blood through a placenta. The offspring develops in the female body and is born alive. Mammals, some cartilaginous fish, and a few reptiles are some viviparous animals.

Additional information: Some advantages of internal fertilization over external fertilization are as follows:
- Internal fertilization protects the fertilized egg or embryo from any foreign organisms or predators and brutal conditions. This ensures improved survival rates of the embryo than can occur with external fertilization.
- Greater chance of successful fertilization
- Less chance of desiccation of gametes

Note: After implantation of sperm in the ovum, the cortical granules within the oocyte fuse with the plasma membrane of the cell, and are expelled into the zona pellucida, causing the surface to become hard and impenetrable. The process is termed as the cortical reaction and is responsible for ensuring that only one single sperm cell can enter and fertilize the egg.