B. Internal Ferilization and external Development: This mode is common in birds, lizards, reptiles etc where fertilization occurs inside and then development occur outside. In this mode, sperms are inserted into the female body through an appropriate organ. After fertilization, zygote passes outside the female reproductive tract and develop into the offsprings.

C. Internal Ferilization and Internal Development: This mode is common in mammals where sperms are inserted into the female body through an appropriate organ. After fertilization, embryo develop into the offsprings.

3. Post-fertilization events: All events after fertilization are considered as post-fertilization events. These events are as follows:

A. Zygote formation: Fusion of male and female gametes give rise to diploid (2n) zygote. Formation of zygote is the first step towards development of offsprings.

B. Embryogenesis: Zygote develops into the embryo through a series of events collectively known as embryogenesis. In few animals, embryogenesis can occur inside the female reproductive system whereas in few cases it occurs outside the female body. Based on the embryogeneis, animals are classified into two classes:

 
(i) Oviparous Animals: Animals lay fertilized eggs or infertilized eggs. The animals fall under this category are birds and reptiles. The egg has hard calcareous shell to protect from damage. The eggs are incubated at a particular temperature and ultimately offsprings hatch out from the egg.

(ii) Viviparous Animals:  These animals give birth of young ones. The zygote develops inside the body of female.