Q.99 The embryos of which one or more of the following animals show meroblastic
cleavage?
(A) Danio rerio (zebrafish)
(B) Gallus gallus (chicken)
(C) Synapta digita (sea cucumber)
(D) Xenopus laevis (frog)

Danio rerio (zebrafish) and Gallus gallus (chicken) embryos exhibit meroblastic cleavage. This partial cleavage pattern occurs in eggs with large yolk reserves, where cell divisions are restricted to the cytoplasmic region. The correct answer is (A) and (B).

Option Analysis

Danio rerio (zebrafish): Zebrafish eggs are telolecithal with abundant yolk, leading to discoidal meroblastic cleavage confined to the blastodisc at the animal pole. Cell divisions do not penetrate the yolk mass.​

Gallus gallus (chicken): Chicken eggs undergo discoidal meroblastic cleavage, where divisions occur only in the blastodisc atop the massive yolk, forming a flattened blastoderm.​

Synapta digita (sea cucumber): As an echinoderm, Synapta digita shows holoblastic radial cleavage in its microlecithal egg, with complete division through the entire zygote.​

Xenopus laevis (frog): Frog embryos display holoblastic cleavage, though unequal due to moderate yolk; divisions fully bisect the egg despite yolk distribution.​

Meroblastic cleavage Danio rerio Gallus gallus represents a key embryonic development pattern in yolky eggs, vital for CSIR NET life sciences preparation. This article details how Danio rerio (zebrafish) and Gallus gallus (chicken) embryos undergo partial cell divisions, contrasting with holoblastic cleavage in Synapta digita (sea cucumber) and Xenopus laevis (frog).

Cleavage Types Overview

Cleavage divides the zygote into blastomeres, classified as holoblastic (complete) or meroblastic (partial) based on yolk amount. Meroblastic cleavage Danio rerio Gallus gallus occurs in telolecithal eggs, restricting divisions to cytoplasm.​

• Holoblastic: Full egg division (e.g., microlecithal eggs).

• Meroblastic: Discoidal (blastodisc) or superficial subtypes.​

Zebrafish Embryo Cleavage

Danio rerio embryos initiate meroblastic discoidal cleavage 45 minutes post-fertilization in the yolk-free blastodisc. Synchronous divisions form a cap-like embryo atop undivided yolk.​

Chicken Embryo Cleavage

Gallus gallus features extreme telolecithal eggs with meroblastic discoidal cleavage in the blastodisc, yielding a blastoderm that spreads over yolk.​

Non-Meroblastic Options

Synapta digita (echinoderm) undergoes holoblastic radial cleavage, fully partitioning its small egg. Xenopus laevis shows holoblastic unequal cleavage, distributing yolk-laden cells.​