Question

During early embryonic development in humans, which of the following options shows the correct sequence of the various events given below.
I. Formation of hypoblast and epiblast
II. Appearance of primitive streak
III. Differentiation of cytotrophoblast and syncytiotrophoblast.
IV. Formation of amnion

• A. III \(\rightarrow\) I \(\rightarrow\) IV \(\rightarrow\) II
• B. II \(\rightarrow\) III \(\rightarrow\) IV \(\rightarrow\) I
• C. I \(\rightarrow\) III \(\rightarrow\) II \(\rightarrow\) IV
• D. IV \(\rightarrow\) I \(\rightarrow\) III \(\rightarrow\) II

Hint:

Trophoblast differentiation comes first, and gastrulation with the primitive streak comes last.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept
At implantation (about day 7) the trophoblast differentiates first.

Step 2: Order of events
III: trophoblast splits into cytotrophoblast and syncytiotrophoblast.
I: the inner cell mass forms a disc of epiblast and hypoblast.
IV: a cavity in the epiblast gives the amnion.
II: gastrulation begins in the third week with the primitive streak.

Step 3: Conclusion
The sequence is III, I, IV, II.

Final Answer:
The sequence is III, I, IV, II. \[ \boxed{\text{(A) }\text{III}\rightarrow\text{I}\rightarrow\text{IV}\rightarrow\text{II}} \]