If the first cleavage furrow divides zygote completely into two, the cleavage type is

The correct option is holoblastic.

To understand why this is the correct answer, let us break down the concept of cleavage in developmental biology step-by-step:

1. Definition of Cleavage:
Cleavage is the series of rapid mitotic cell divisions that a fertilized egg (zygote) undergoes to transform into a multicellular structure. The divisions are characterized by the formation of cleavage furrows, which are indentations in the cell surface that deepen to divide the cell.

2. Types of Cleavage based on Fate of the Zygote:
Depending on the amount and distribution of yolk in the egg, cleavage can be categorized into two primary types:

• Holoblastic Cleavage (Complete Cleavage): In this type of cleavage, the cleavage furrows pass completely through the zygote, dividing it entirely into separate, distinct cells (blastomeres). This typically occurs in eggs with little to no yolk (isolecithal or mesolecithal eggs), allowing the furrow to penetrate all the way through.

• Meroblastic Cleavage (Incomplete/Partial Cleavage): Here, the cleavage furrows do not penetrate the entire egg because of a large accumulation of yolk. Only the cytoplasmic region undergoes division, leaving the yolk undivided.

3. Why the other options are incorrect:
• meroblastic: As explained, this refers to incomplete division, which contradicts the prompt stating the zygote is divided completely.
• equatorial: This refers to the plane or direction of a specific cleavage division (horizontal or along the equator), not the general type of cleavage based on completeness.
• radial: This describes the spatial arrangement or symmetry of the blastomeres relative to the polar axis, rather than whether the division of the zygote is complete or partial.

Therefore, because the first cleavage furrow divides the zygote completely into two, the cleavage type is holoblastic.