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:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (C), (A), (B), (D)
Germ layers form first, in the early embryo (C); the heart forms after one month, and heart sounds can be heard (A); limbs and digits develop by the end of the second month (B); eyelids separate by the end of 24 weeks, in the second trimester (D).
NCERT Extract:
Pregnancy and Embryonic Development – “… 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.”