Arrange the following events chronologically as they occur during embryonic development. 

(A) Appearance of heart sounds
(B) Development of limbs and digits
(C) Formation of three primary germ layers
(D) Separation of eyelids

Choose the correct answer from the options given below:

(C), (A), (B), (D)

The correct answer is Option (2) → (C), (A), (B), (D)

(C) Formation of three primary germ layers

(A) Appearance of heart sounds

(B) Development of limbs and digits

(D) Separation of eyelids

Immediately after implantation, the inner cell mass (embryo) differentiates into an outer layer called ectoderm and an inner layer called endoderm. A mesoderm soon appears between the ectoderm and the endoderm. These three layers give rise to all tissues (organs) in adults.

In human beings, after one month of pregnancy, the embryo’s heart is formed. The first sign of growing foetus may be noticed by listening to the heart sound carefully through the stethoscope. 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, for example, the limbs and external genital organs are well- developed. The first movements of the foetus and appearance of hair on the head are usually observed during the fifth month. By the end of about 24 weeks (end of second trimester), the body is covered with fine hair, eye-lids separate, and eyelashes are formed. By the end of nine months of pregnancy, the foetus is fully developed and is ready for delivery.