Observe the following figures. (i) Identify the stages a to d in above figures during development of human baby. (ii) Arrange the stages in correct sequence of development. (iii) Explain the development that takes place in any one stage.
(a) (b) (c) (d)

(i) a. Embedding of the embryo in the uterus.

b. Fertilisation.

 c. Zygote formation and development of an embryo from the zygote.

d. Zygote showing fusion of nuclei.

(ii) The correct sequence is (b), (d), (c), (a)

(iii) Zygote formation: The sperm and the egg nuclei fuses to form a single nucleus resulting in the formation of a fertilised egg or zygote.