(b) : In human beings, after one month of pregnancy, the embryo’s heart is formed. By the end of the second month of pregnancy, the foetus develops limbs and digits. By the end of $12$ weeks (first trimester), most of the major organ systems are formed. The first movements of the foetus and appearance of hair on the head are usually observed during the fifth month. By the end of $24$ weeks (second trimester), the body is covered with fine hair, eye­lids separate, and eyelashes are formed. By the end of nine months of pregnancy, the foetus is fully developed and is ready for delivery.