Observe the following figures.

(a)

(b)

(c)

(d)

(i) Identify the stages a to d in Fig. 9.4 during the development of the human baby.

(ii) Arrange the stages in the correct sequence of development.

(iii) Explain the development that takes place in any one stage.

(i) The stages are as follows:

a. Embedding of the embryo in the uterus of the mother, i.e. implantation.

b. Fusion of male and female gametes, i.e. fertilization.

c. Formation of the zygote and it’s development into an embryo as it travels from oviducts to uterus.

d. Fusion of nuclei during fertilization, in a zygote.

(ii) The correct sequence of these stages would be :

1. b. Fusion of male and female gametes, i.e. fertilization.

2. d. Fusion of nuclei of two gametes in a zygote.

3. c. Development of a zygote to an embryo as it travels down the oviducts to reach the uterus.

4. a. Embedding of the embryo in uterine walls of the mother, i.e. implantation.

Thus, the correct sequence would be: b, d, c, a.

(iii) During fertilization, the male gamete sperm reaches up to the female gamete, ovum or the egg cell. One of the sperms that reach the ovum, releases its nuclei into the egg cell. Then, inside the egg cell, or the zygote, takes place the fusion of nucleus of that sperm with the nucleus of the female gamete. Thus, the product of fertilization, the zygote is formed. The rest of the sperm degenerates.