The first cavity that develops in an embryo is known as what?

The correct answer is the coelomic cavity, which is the first body cavity that forms during embryonic development. It arises from the mesoderm layer after the initial formation of the embryo. This cavity plays a crucial role in the development of the organism, as it will eventually evolve into spaces that house the internal organs and serve multiple physiological functions.

In relation to the other options, the amniotic sac is not the first cavity to form; rather, it develops later during the embryonic stage to surround and protect the developing embryo. The chorion is also derived from trophoblastic tissue and does not represent a cavity but rather a membrane that forms part of the placenta. The blastocyst is an early stage of embryonic development that eventually implants into the uterine wall, but it is not a cavity; instead, it is a hollow structure containing the inner cell mass that will develop into the embryo itself. Therefore, the coelomic cavity stands out as the correct answer in terms of embryonic development specifics.