Which of the following is the first to appear during pre-natal development in human beings?

The embryo’s heart is formed after one month of pregnancy. The foetus develops limbs and digits by the end of the second month of pregnancy.  The limbs and external genital organs are well-developed by the end of first trimester. Appearance of hair on the head is usually observed during the fifth month. The body is covered with fine hair by the end of the second trimester.