CBSE Class 10 Answered

Organisms exhibiting external fertilisation show great synchrony between the sexes and release a large number of gametes into the surrounding medium in order to enhance the chances of fertilisation. Many a times, the gametes are covered with protective coverings to enhance their survival in the environment. For example in several organisms like birds and reptiles, the ovum is protected in a hard outer shell for its continued survival after fertilization. Some spores like those of moss can germinate after being dessicated for a long time.

The female gamete is usually large since it has more food stored in it . This is so that it can support the complete development of the embryo right upto maturity. Also it will have some mechanism to prevent the embryo from drying. Different types of eggs and shells are found to enable them to survive externally. The egg shells will have tiny pores to allow the embryo to breathe. The male gamete on the other hand, will be much smaller, since it requires only some food to enable it to swim and reach the female gamete.

Fish eggs are fertilized externally, typically with the male inseminating the eggs after the female lays them. These eggs do not have a shell and would dry out in the air. Even air-breathing amphibians lay their eggs in water, or in protective foam. Reptile eggs are often rubbery and are able to survive in air.