Q.88 During development of which ONE of the following organisms, bilateral meroblastic cleavage is
found?
(A) Mollusc (B) Fish (C) Bird (D) Amphibian
Bilateral meroblastic cleavage questions test egg yolk distribution and cleavage patterns in embryology. Q.88 identifies which organism shows this specific cleavage type among Mollusc, Fish, Bird, and Amphibian. The correct answer is (A) Mollusc.
✅ Correct Answer: (A) Mollusc
Bilateral meroblastic cleavage = Partial cleavage (meroblastic) that divides egg into mirror-image left/right halves (bilateral).
Molluscs (squid, octopus, bivalves):
Yolk-rich telolecithal egg → Cleavage restricted to animal pole
First cleavage: Vertical (meridional) → 2 equal blastomeres
Second cleavage: Vertical (90° to first) → 4 equal blastomeres (bilateral symmetry)
Cleavage doesn't penetrate yolk mass
❌ Cleavage Patterns: All Options Explained
| Animal | Egg Type | Cleavage Type | Pattern | Why NOT Bilateral Meroblastic |
|---|---|---|---|---|
| (A) Mollusc | Telolecithal (yolk at vegetal pole) | Bilateral Meroblastic | Left/right mirror cleavage | ✅ CORRECT |
| (B) Fish | Telolecithal | Discoidal Meroblastic | Blastodisc on yolk surface | Circular, not bilateral |
| (C) Bird | Macrolecithal (huge yolk) | Discoidal Meroblastic | Blastodisc forms | Circular cleavage on yolk |
| (D) Amphibian | Mesolecithal (moderate yolk) | Holoblastic (complete) | Equal blastomeres | Complete division, not partial |
🔬 Cleavage Pattern Visual Guide
MOLLUSC (Bilateral Meroblastic):
Animal Pole
┌─────────────┐
│ Left │ Right │ ← Bilateral symmetry
│ Blastomere│Blastomere│
└──────┬────┘
│ Yolk (uncleaved)
▼
FISH/BIRD (Discoidal Meroblastic):
Blastodisc (circular)
┌──────┐
│ │ ← Circular
│Blast│
└──────┘
│
Massive Yolk
🎯 Exam Memory Framework: Yolk → Cleavage Type
| Yolk Amount | Cleavage | Examples |
|---|---|---|
| Little yolk | Holoblastic | Mammals, Amphibians |
| Moderate yolk | Holoblastic unequal | Amphibians |
| Much yolk (vegetal) | Meroblastic discoidal | Fish, Birds, Reptiles |
| Much yolk (vegetal) + Bilateral | Meroblastic bilateral | Molluscs, Annelids |
Quick ID: BILATERAL = MOLLUSCS (squid/octopus embryos show perfect left/right symmetry)
Why Other Options Fail
(B) Fish – Discoidal meroblastic
-
Cleavage forms blastodisc (circular cap) on yolk surface
-
No left/right mirror cleavage
(C) Bird – Discoidal meroblastic
-
Meroblastic (partial) but discoidal (circular blastodisc)
-
Yolk too massive for bilateral pattern
(D) Amphibian – Holoblastic
-
Complete cleavage through entire egg
-
Smaller, faster blastomeres (no partial cleavage)
NEET/GATE Pattern Recognition
Typical question traps:
"Meroblastic" → Eliminate amphibians (holoblastic)
"Discoidal" → Fish/Birds
"Bilateral" → Molluscs only
Master bilateral meroblastic cleavage – this exact question repeats across exams!
One-word answer: MOLLUSC (A) – Only molluscs show bilateral meroblastic cleavage among given options.
