Which of the following develops into pseudocoelom?

The correct option is Blastocoel.

To understand why the blastocoel develops into a pseudocoelom, we can look at the embryonic development of triploblastic animals (animals with three germ layers: ectoderm, mesoderm, and endoderm).
During the blastula stage of embryonic development, a fluid-filled cavity forms within the sphere of cells. This cavity is called the blastocoel.

In pseudocoelomate animals (such as roundworms or nematodes), the blastocoel is not completely obliterated or replaced by a true coelom lined by mesoderm. Instead, the blastocoel persists into the adult stage, remaining as a cavity located between the gut (derived from endoderm) and the body wall (derived from ectoderm). Because this cavity is not fully lined by mesodermal epithelium, it is referred to as a "false coelom" or pseudocoelom.

Let us evaluate the other options to understand why they are incorrect:
1. Archenteron: This is the embryonic gut cavity formed during gastrulation, which eventually develops into the adult digestive tract (gut lumen).
2. Blastopore lip: This is the rim of the blastopore (the opening of the archenteron) and is an organizing tissue region during gastrulation, not a cavity that becomes a coelom.
3. Embryonic mesoderm: This is the germ layer that gives rise to muscles, connective tissues, and the lining of a true coelom (eucoelom), but it does not directly form the pseudocoelom cavity itself.