Q72.Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A :
Primitive groove is the functional equivalent to dorsal lip of the amphibian blastopore.
Reason R :
Dorsal lip of the amphibian blastopore is considered as the primary organiser.
In the light of the above statements, choose the most appropriate answer from the options given below :
(1) Both A and R are correct and R is the correct explanation of A
(2) Both A and R are correct but R is not the correct explanation of A
(3) A is correct but R is not correct
(4) A is not correct but R is correct
The correct answer is option (1): Both A and R are correct and R is the correct explanation of A.
Statement Analysis
Assertion A correctly states that the primitive groove (in bird/chick embryos) functions equivalently to the dorsal lip of the amphibian blastopore, as both serve as sites of cell ingression during gastrulation and initiate body axis formation through organizer activity. Reason R is true—the dorsal lip, discovered by Spemann and Mangold, is the classic “primary organizer” that induces neural tissue and patterns the embryo when transplanted. R explains A because the equivalence stems from shared organizer function: both regions emit signals (e.g., Chordin, Noggin) that dorsalize mesoderm and induce ectoderm to neural fate.
Option Breakdown
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Option (1): Both true, R explains A—primitive groove = avian homolog of amphibian organizer (dorsal lip).
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Option (2): Both true but unrelated—wrong, as equivalence derives directly from organizer role.
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Option (3): A true, R false—wrong, dorsal lip is definitively the primary organizer.
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Option (4): A false, R true—wrong, homology between primitive groove and dorsal lip is established embryology.
Introduction: Primitive Groove Functional Equivalent Dorsal Lip Amphibian Blastopore
NEET developmental biology tests embryonic organizers via assertions like “primitive groove is the functional equivalent to dorsal lip of the amphibian blastopore.” Both as primary organizers (option 1) links amphibian and avian gastrulation. This solves A+R, Spemann-Mangold legacy, and exam strategy.
Dorsal Lip: Primary Organizer Role
In amphibians (Xenopus), dorsal blastopore lip involutes first, secreting BMP inhibitors to:
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Induce neural plate from ectoderm.
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Pattern notochord/somite axis.
Transplant experiments yield secondary embryos, proving “organizer” status.
Primitive Groove: Avian Counterpart
Bird primitive streak’s groove enables epiblast ingress to hypoblast, forming mesendoderm. Hensen’s node (streak tip) mirrors dorsal lip—both:
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Express homologous genes (Brachyury, Goosecoid).
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Drive axis via Wnt/Chordin gradients.
Why Option (1): R Explains A Perfectly
Equivalence = shared induction (neural, axial). Not mere morphology—functional homology via signaling.
