Cleavage in Eggs: Holoblastic vs Meroblastic

Quick answer: Cleavage is the first stage of embryonic development: the zygote divides repeatedly by mitosis into smaller cells (blastomeres) without growing. The amount and distribution of yolk decides the pattern: little yolk gives complete (holoblastic) cleavage, abundant yolk gives partial (meroblastic) cleavage, forming first the morula, then the hollow blastula.

Key Facts: Cleavage

• Cleavage is the series of rapid mitotic divisions of the zygote into smaller cells (blastomeres), with no growth between divisions. The embryo gets divided up, not bigger.
• Yolk calls the shots: its amount and distribution decide the cleavage type. Little yolk gives holoblastic (complete) cleavage; lots of yolk gives meroblastic (partial) cleavage.
• Holoblastic cleavage is equal (amphioxus, mammals) or unequal (frog, with micromeres and macromeres).
• Meroblastic cleavage is discoidal (birds, reptiles, fish) or superficial (insects).
• Cleavage first builds the morula (a solid ball of cells) and then the blastula (a hollow ball with a blastocoel inside).
• Determinate cleavage (spiral; protostomes) fixes each blastomere's fate early; indeterminate cleavage (radial; deuterostomes) lets each early blastomere form a complete embryo: the basis of identical twins.

Here is something surprising: after fertilization, the zygote does not grow. Not one bit. It divides, and divides again, into smaller and smaller cells, while the whole embryo stays exactly the size of the original egg. This rapid chopping-up process is called cleavage, and it is the first chapter of every animal's development. Now here is the question that unlocks the whole topic: why does a frog's egg divide completely, while a hen's egg divides only in one tiny disc? One word: yolk. Once you see how yolk controls cleavage, this topic turns into easy marks.

What Is Cleavage? Special Features

Isn't this just ordinary mitosis? Yes, but with house rules. Cleavage differs from ordinary cell division in several ways:

• No growth: blastomeres get smaller with each division, and the embryo stays the same size as the zygote. (A common exam mistake is writing that cleavage increases the size of the embryo. Don't make that mistake.)
• Rapid: the cell cycles are extremely short, basically just S and M phases one after another, which restores the nucleocytoplasmic ratio.
• Product: the divisions first form a solid ball of 16 to 64 cells, the morula, then a hollow ball, the blastula, enclosing a cavity called the blastocoel.
• Cleavage planes follow rules (Sachs' rules): each plane tends to pass through the animal-vegetal axis and at right angles to the previous one.

Various stages of cleavage in the frog egg, from the first furrow to the segmenting ovum. Image: Wesley Mills (after Howes), public domain, via Wikimedia Commons.

How Yolk Determines Cleavage Type

Think of yolk as a roadblock. It is inert food material, and the cleavage furrow simply cannot cut through it easily. So the amount of yolk, and where it sits in the egg, decides everything. Based on yolk, eggs fall into four groups:

• Microlecithal / isolecithal: little, evenly distributed yolk (e.g. amphioxus, mammals).
• Mesolecithal: moderate yolk, concentrated toward the vegetal pole (e.g. frog).
• Macrolecithal / telolecithal: abundant yolk at the vegetal pole (e.g. birds, reptiles, bony fishes).
• Centrolecithal: yolk concentrated in the center (e.g. insects).

Master Table: Egg Type, Yolk and Cleavage

Egg type	Yolk distribution	Cleavage type	Example
Microlecithal / isolecithal	Little yolk, evenly distributed	Holoblastic, equal	Amphioxus, echinoderms, placental mammals (including humans)
Mesolecithal	Moderate yolk, concentrated at the vegetal pole	Holoblastic, unequal	Frog and other amphibians
Macrolecithal / telolecithal	Abundant yolk at the vegetal pole	Meroblastic, discoidal	Birds, reptiles, bony fishes
Centrolecithal	Yolk concentrated in the center	Meroblastic, superficial	Insects (e.g. Drosophila)

Classification of Cleavage: A Hierarchical View

Every cleavage pattern fits into one nested classification. Learn it top-down:

• Holoblastic cleavage (complete: the furrow cuts through the entire egg; little yolk)
  • Equal holoblastic: all blastomeres roughly equal in size
    • Amphioxus, echinoderms, placental mammals (including humans; also rotational in pattern)
  • Unequal holoblastic: small micromeres at the animal pole + large macromeres at the vegetal pole
    • Frog and other amphibians (third furrow displaced from the equator by vegetal yolk)
• Meroblastic cleavage (partial: the furrow cannot cut through the massive yolk; only active cytoplasm divides)
  • Discoidal: division restricted to a small cytoplasmic disc (germinal disc) at the animal pole, sitting on the yolk
    • Birds, reptiles, bony fishes, egg-laying mammals
  • Superficial: division restricted to the peripheral cytoplasm around central yolk; nuclei divide first, then migrate outward
    • Insects (e.g. Drosophila)

Holoblastic (Complete) Cleavage

Here's a memory hook: holo means whole. The cleavage furrow cuts through the entire egg. Learn the two subtypes as a pair:

Type	Blastomeres	Egg type	Examples
Equal holoblastic	All blastomeres roughly equal in size	Microlecithal / isolecithal	Amphioxus, echinoderms, placental mammals
Unequal holoblastic	Small micromeres (animal pole) + large macromeres (vegetal pole)	Mesolecithal	Frog and other amphibians

Now watch what the yolk does in the frog: the third cleavage furrow gets pushed away from the equator by the vegetal yolk, so you end up with 4 small micromeres at the animal pole and 4 large macromeres at the vegetal pole. Small cells on top, big yolky cells below. That one picture is unequal holoblastic cleavage.

Meroblastic (Partial) Cleavage

Mero means part. The furrow cannot cut through the massive yolk, so only the active cytoplasm divides:

Type	Where division occurs	Egg type	Examples
Discoidal	Restricted to a small cytoplasmic disc (germinal disc) at the animal pole, sitting on the yolk	Telolecithal (macrolecithal)	Birds, reptiles, bony fishes, egg-laying mammals
Superficial	Restricted to the peripheral cytoplasm around central yolk; nuclei divide first, then migrate outward	Centrolecithal	Insects (e.g. Drosophila)

Discoidal cleavage in a zebrafish egg: the 1-cell, 2-cell, 4-cell and 8-cell stages, with division restricted to the blastodisc atop the yolk. Image: S. F. Gilbert, CC BY-SA 4.0, via Wikimedia Commons.

Cleavage Patterns (Geometry of Division)

So far we have talked about how much of the egg divides. Now the next question: in what geometry? Four classic patterns, and yes, examiners love all four:

• Radial cleavage: blastomeres sit directly above one another (echinoderms, amphibians); typical of deuterostomes.
• Spiral cleavage: blastomeres divide at an angle, offset from the tier below (molluscs, annelids); typical of protostomes.
• Bilateral cleavage: divisions produce mirror-image halves (ascidians/tunicates).
• Rotational cleavage: the first division is meridional, the second rotates 90 degrees in one blastomere (placental mammals, including humans).

One more distinction, and it is a favorite trap: determinate vs indeterminate. In determinate cleavage (spiral; protostomes) each blastomere's fate is fixed early. In indeterminate cleavage (radial; deuterostomes) early blastomeres can each still form a complete embryo. And that, by the way, is exactly how identical twins happen.

What Comes After Cleavage?

Cleavage ends with the blastula: a hollow ball of cells around the blastocoel. The next chapter is gastrulation, which reorganizes the blastula into the three-layered gastrula (ectoderm, mesoderm, endoderm) with the archenteron. In amniotes, the embryo then builds the extraembryonic membranes (amnion, chorion, allantois, yolk sac). The full early-development sequence is therefore: fertilization, cleavage, blastula, gastrulation, gastrula, neurulation.

Examiner tip

If you carry only one line out of this topic, make it this: "In an egg, the type of cleavage is determined by the amount and distribution of yolk." That single fill-in-the-blank shows up almost every year. Then memorize these four pairs: equal holoblastic, amphioxus and mammals; unequal holoblastic, frog; discoidal, birds and reptiles; superficial, insects. Quick self-check: discoidal + superficial = meroblastic. And radial cleavage marks deuterostomes, while spiral marks protostomes.

Frequently Asked Questions

What is the real difference between holoblastic and meroblastic cleavage?

In holoblastic cleavage the furrow divides the entire egg (eggs with little yolk: amphioxus, mammals, frog). In meroblastic cleavage the furrow cannot pass through the yolk, so division is partial: restricted to the germinal disc (discoidal: birds, reptiles) or to the surface cytoplasm (superficial: insects).

What type of cleavage do humans have?

Humans (placental mammals) show equal holoblastic, rotational cleavage: the nearly yolk-free (microlecithal) egg divides completely, with the second division rotated 90 degrees relative to the first.

I keep mixing up micromeres and macromeres. Any trick?

Think size: micro = small, macro = large. In unequal holoblastic cleavage (e.g. frog), the yolk-rich vegetal half divides slowly, producing large yolk-laden macromeres, while the animal half divides fast, producing small micromeres.

Morula vs blastula, aren't they the same thing?

Not quite. The morula is the solid ball of blastomeres (16 to 64 cells) formed by early cleavage; the blastula is the later hollow ball with a fluid-filled cavity, the blastocoel.

Why is spiral cleavage always paired with protostomes?

Spiral cleavage (offset, determinate divisions) is characteristic of molluscs and annelids, the protostome lineage, whereas radial, indeterminate cleavage is characteristic of echinoderms and chordates, the deuterostomes. It is a favorite MDCAT comparison.

What comes after cleavage in embryonic development?

Gastrulation comes next: it reorganizes the hollow blastula into the three-layered gastrula (ectoderm, mesoderm, endoderm) with a new cavity, the archenteron. The full sequence is fertilization, cleavage, blastula, gastrulation, gastrula, neurulation.

Practice MCQs

Q1. The type of cleavage in an egg is determined by:

1. Size of the sperm
2. Amount and distribution of yolk
3. Number of chromosomes
4. Temperature of the environment

Answer: B

Q2. Superficial cleavage is found in:

1. Frog
2. Birds
3. Insects
4. Mammals

Answer: C

Q3. Discoidal cleavage occurs in the eggs of:

1. Amphioxus
2. Frog
3. Hen
4. Human

Answer: C

Q4. In unequal holoblastic cleavage of the frog egg, the smaller blastomeres are called:

1. Macromeres
2. Micromeres
3. Blastomeres
4. Trophomeres

Answer: B

Q5. Which cleavage pattern is characteristic of deuterostomes?

1. Spiral, determinate
2. Radial, indeterminate
3. Superficial
4. Discoidal

Answer: B

Q6. The blastocoel is:

1. The solid ball of blastomeres formed by early cleavage
2. The yolk-filled vegetal half of the frog egg
3. The germinal disc of a bird egg
4. The cavity enclosed by the hollow blastula

Answer: D

Q7. In the frog egg, the third cleavage furrow is displaced from the equator because:

1. Vegetal yolk resists the cleavage furrow
2. The animal pole divides more slowly
3. The sperm entry point shifts the plane
4. Nuclei migrate outward before dividing

Answer: A

Q8. Superficial cleavage in insects is distinctive because:

1. The furrow cuts through the entire egg
2. Division is restricted to the germinal disc at the animal pole
3. The blastomeres remain a solid morula
4. Nuclei divide first and then migrate to the peripheral cytoplasm

Answer: D

Q9. Identical twins are possible because mammalian cleavage is:

1. Determinate, fixing each blastomere's fate early
2. Spiral, with offset blastomere tiers
3. Indeterminate: early blastomeres can each form a complete embryo
4. Discoidal, confined to the germinal disc

Answer: C

Q10. Rotational cleavage of placental mammals is characterized by:

1. The first division meridional and the second rotated 90 degrees in one blastomere
2. Blastomeres stacked directly above one another
3. Divisions at an angle offset from the tier below
4. Division restricted to a small cytoplasmic disc at the animal pole

Answer: A

Q11. The germinal disc, where discoidal cleavage occurs, is located at the:

1. Vegetal pole
2. Animal pole
3. Center of the yolk
4. Equator of the egg

Answer: B

Q12. Cleavage restores the nucleocytoplasmic ratio because:

1. The embryo grows rapidly between divisions
2. Rapid divisions without growth make the cells progressively smaller
3. The yolk is digested during each division
4. Nuclei migrate outward before dividing

Answer: B

Short Questions

Q1. What determines the type of cleavage in an egg? See answer

The amount and distribution of yolk determine the cleavage type. Yolk is inert food material that resists the cleavage furrow: where yolk is abundant, the furrow cannot cut through it, so division is restricted or displaced.

With little yolk (microlecithal/isolecithal or mesolecithal eggs), the furrow cuts the whole egg: holoblastic (complete) cleavage, as in amphioxus, mammals and the frog. With massive yolk (telolecithal or centrolecithal eggs), the furrow cannot pass through: meroblastic (partial) cleavage, as in birds, reptiles and insects.

This is the single most-asked exam question on the topic, usually as a fill-in-the-blank: "In an egg, the type of cleavage is determined by _____." Answer: the amount and distribution of yolk.

Q2. Differentiate between holoblastic and meroblastic cleavage with examples. See answer

In holoblastic cleavage (holo = whole), the cleavage furrow cuts through the entire egg. It is equal when blastomeres are all the same size (microlecithal eggs: amphioxus, echinoderms, placental mammals) and unequal when vegetal yolk slows division (mesolecithal eggs: the frog, with micromeres and macromeres).

In meroblastic cleavage (mero = part), the furrow cannot cut through the massive yolk, so only the active cytoplasm divides. It is discoidal when division is restricted to the germinal disc at the animal pole (telolecithal eggs: birds, reptiles, bony fishes) and superficial when restricted to the peripheral cytoplasm (centrolecithal eggs: insects).

The contrast to memorize: holoblastic = whole egg divides (little yolk); meroblastic = partial division (much yolk). Discoidal + superficial = meroblastic, a favourite true/false trap.

Q3. What are micromeres and macromeres? Where are they found? See answer

Micromeres are the small blastomeres and macromeres the large ones (micro = small, macro = large). They are found in unequal holoblastic cleavage, the classic example being the frog egg.

Because yolk is concentrated at the vegetal pole, the vegetal half divides slowly and produces large, yolk-laden macromeres, while the yolk-poor animal half divides rapidly and produces small micromeres. The third cleavage furrow is displaced from the equator by vegetal yolk, giving exactly 4 animal micromeres and 4 vegetal macromeres.

This animal-vegetal size difference sets up the polarity of the blastula (animal hemisphere vs vegetal hemisphere), which later determines where gastrulation begins: at the marginal zone between them.

Q4. What is the difference between the morula and the blastula? See answer

The morula is the solid ball of blastomeres (16-64 cells) formed by the early rounds of cleavage; "morula" means "little mulberry", which it resembles. There is no internal cavity yet.

The blastula is the later hollow ball of cells enclosing a fluid-filled cavity, the blastocoel (segmentation cavity). The transition from morula to blastula marks the end of cleavage proper.

Remember the products in order: cleavage, morula (solid), blastula (hollow, with blastocoel). Cleavage itself involves no growth: the blastomeres simply get smaller while the embryo stays the size of the zygote, restoring the nucleocytoplasmic ratio.

Q5. Which cleavage patterns mark protostomes and deuterostomes? See answer

Protostomes (e.g. molluscs, annelids) show spiral, determinate cleavage: blastomeres divide at an angle, offset from the tier below, and each blastomere's developmental fate is fixed early. Remove one blastomere and the embryo cannot replace it.

Deuterostomes (e.g. echinoderms, chordates) show radial, indeterminate cleavage: blastomeres sit directly above one another, and early blastomeres each retain the capacity to form a complete embryo. This is exactly how identical twins arise.

The pairing to memorize is spiral-determinate-protostome versus radial-indeterminate-deuterostome. It is a favourite MDCAT comparison and also explains why the frog's blastopore becomes the anus (deuterostome pattern).

Q6. What type of cleavage occurs in the human egg? See answer

Humans (placental mammals) show equal holoblastic, rotational cleavage. The egg is nearly yolk-free (microlecithal), so the furrow cuts through the entire egg completely, producing roughly equal blastomeres.

The rotational part is the distinctive geometry: the first division is meridional, while the second division is rotated 90 degrees in one of the two blastomeres. This pattern is unique to placental mammals.

Because the cleavage is indeterminate, each early blastomere can still form a complete embryo: the biological basis of identical twins, which arise when the early embryo splits in two.

References

1. Hickman, C.P., Roberts, L.S. and Larson, A. Integrated Principles of Zoology, 18th ed. McGraw-Hill. (Early development; cleavage patterns.)
2. Miller, S.A. and Harley, J.P. Zoology, McGraw-Hill. (Cleavage and blastula formation.)
3. OpenStax, Biology 2e, Chapter 43: Animal Reproduction and Development: https://openstax.org/details/books/biology-2e.
4. NCBI Bookshelf: Developmental Biology, Cleavage: https://www.ncbi.nlm.nih.gov/books/.

Read next

• Gastrulation in Frog: Stages and Blastopore
• Extraembryonic Membranes: Amnion and Chorion
• Spermatogenesis vs Oogenesis Differences