List all the changes that occur in the ovum during fertilisation.

Before the event of fertilization, the egg stays in the quiescent state i.e. in the metaphase of the second meiotic division.
The process of the fusion of the male and the female gamete resulting in the formation of zygote is known as fertilization.
As soon as the sperm binds with the egg, the egg rapidly shows a number of metabolicand physical changes. These changes are often referred to as egg activation. The changes which take place in the egg during
fertilization are as follows:
1) Intracellular concentration of calcium rises.
2) The second meiotic division gets completed.
3) Completion of the cortical reaction.
The cortical reaction may be defined as a massive exocytosis of cortical granules which are observed shortly after the sperm fuses with the oocyte. These cortical granules contains various enzymes such as several proteases which in turn diffuse into the zona pellucida after exocytosis from the egg. These proteases then changes the structure of zona pellucida resulting in the zona reaction.

The zona reaction may be defined as the process catalyzed by proteases from cortical granules which results in the alteration in the structure of the zona pellucida. This reaction is important because it blocks polyspermy.