Discoidal and superficial cleavages are types of

The correct option is Meroblastic.

To understand why this is the correct answer, let us break down the biological concepts surrounding embryonic cleavage:

1. Understanding Cleavage:
Cleavage is the rapid series of mitotic cell divisions that occur after fertilization, transforming the single-celled zygote into a multicellular embryo. The pattern of cleavage is heavily influenced by the amount and distribution of yolk within the egg.

2. Holoblastic vs. Meroblastic Cleavage:
• Holoblastic Cleavage (Complete): In eggs with little to moderate yolk, the cleavage furrows pass completely through the egg, dividing it entirely into blastomeres. This can be equal (producing cells of equal size) or unequal (producing cells of different sizes).
• Meroblastic Cleavage (Incomplete): In eggs containing a massive amount of yolk, the thick yolk restricts the cleavage furrows. As a result, the division cannot penetrate the yolk entirely, and only the active, yolk-free cytoplasmic portion of the egg undergoes division.

3. Types of Meroblastic Cleavage:
Meroblastic cleavage is subdivided based on where the active cytoplasm is located:
• Discoidal Cleavage: The yolk is extremely abundant and fills most of the egg except for a small disc-like cap of active cytoplasm at the animal pole. Cleavage is restricted to this small disc (called the blastodisc). This is commonly observed in birds, reptiles, and fish.
• Superficial Cleavage: The yolk is concentrated in the center of the egg (centrolecithal egg), surrounded by a outer peripheral layer of cytoplasm. The nuclei divide repeatedly in the center and migrate outwards to the surface cytoplasm to form cells. This is characteristic of insects and other arthropods.

Conclusion:
Because both discoidal and superficial cleavages involve incomplete division restricted to a portion of the egg due to high yolk content, they are classified as types of meroblastic cleavage.