ANATOMY

Detailed explanation-1: -The cleavage in the amphioxus egg is of the holoblastic type, and the egg is isolecithal. The egg is divided into two blastomeres around one hour after fertilisation. The egg axis is traversed from pole to pole by the plane of this first cleavage.

Detailed explanation-2: -Assertion: Holoblastic cleavage with almost equal sized blastomeres is a characteristic of placental animals. Reason: Eggs of most mammals, including humans, are of centrolecithal type.

Detailed explanation-3: -Total or holoblastic cleavage-In this type the cleavage furrow bisects the entire egg. Such a cleavage may be either equal or unequal.

Detailed explanation-4: -So, the correct option is ‘Unequal’.

Detailed explanation-5: -Holoblastic cleavage involves the division of the entire egg into blastomeres, and it mainly occurs in isolecithal cells. The patterns followed by these cells during cleavage are radial, spiral, rotational, and bilateral.