Final answer:
During rat embryogenesis, the fluid-filled cavity that develops is the blastocoel, an essential part of the blastocyst which has the inner cell mass fated to become the embryo and the trophoblast intended to form fetal membranes and placenta.
Step-by-step explanation:
The cavity that forms during the embryogenesis in rats and is filled with fluid is most likely the blastocoel. The blastocoel is a fluid-filled cavity of the blastocyst, which is the term for the conceptus at the developmental stage consisting of about 100 cells. This structure is arranged into two primary components: the inner cell mass that is fated to become the embryo, and an outer layer known as the trophoblast that is fated to become the associated fetal membranes and placenta.
The inner cell mass, or embryoblast, consists of a group of cells within the blastocyst that will eventually develop into the embryo. Meanwhile, the blastocoel itself is the cavity that forms by the migration of embryoblast cells to one pole of the blastocyst and fills with fluid secreted by trophoblast cells. The trophoblast, on the other hand, forms the outer cell layer of the blastocyst and will contribute to form the placenta, supporting the embryo's growth.