Q.58 Given below are two statements, one is labelled as
Assertion (A) and the other is labelled as
Reason (R) :
Assertion (A) :
Gastrulation in frog begins at a point on the embryo surface called blastopore.
Reason (R) :
Hensen’s node is the functional equivalent of the dorsal lip of the amphibian blastopore.
In the light of the above statements, choose the
most appropriate answer from the options given below :
- Both (A) and (R) are correct and (R) is the correct explanation of (A)
- Both (A) and (R) are correct but (R) is not the correct explanation of (A)
- (A) is correct but (R) is not correct
- (A) is not correct but (R) is correct
Gastrulation marks a pivotal stage in frog embryonic development, where the single-layered blastula transforms into a triploblastic gastrula via the blastopore. Hensen’s node in higher vertebrates serves as the avian/mammalian counterpart to the frog’s dorsal lip of the blastopore, but these structures operate in distinct contexts.
Correct Answer
Both (A) and (R) are correct but (R) is not the correct explanation of (A).
Assertion (A) Explanation
Gastrulation in frogs indeed begins at the dorsal lip of the blastopore, a key invagination site on the blastula’s vegetal surface opposite the sperm entry point. Cells undergo involution here, displacing the blastocoel and forming the archenteron while establishing germ layers (ectoderm, mesoderm, endoderm). This process starts as a crescent-shaped groove that expands into a ring-shaped blastopore.
Reason (R) Explanation
Hensen’s node is the functional equivalent of the dorsal lip of the amphibian blastopore, acting as an organizer that induces neural tissue and patterns the body axis in chick/mammalian embryos. Both are signaling centers (expressing similar genes like goosecoid, chordin, noggin), but Hensen’s node appears later in bird gastrulation, not at the blastula-to-gastrula transition like the frog blastopore.
Why R Doesn’t Explain A
Assertion (A) describes the spatial/mechanical onset of frog gastrulation at the blastopore. Reason (R) highlights homology between structures across species but doesn’t causally link to why frog gastrulation starts there—it’s a comparative embryology fact, not an explanatory mechanism for the frog process.
Options Breakdown
| Option |
Description |
Correct? |
| Both correct; R explains A |
Implies Hensen’s node directly causes frog blastopore initiation |
❌ No causal link |
| Both correct; R does not explain A |
Accurate statements without R justifying A |
✅ Correct |
| A correct; R incorrect |
Denies established organizer homology |
❌ R is true |
| A incorrect; R correct |
Denies blastopore’s role in frog gastrulation |
❌ A is true |
