Final answer:
An egg cell develops in a follicle and transforms into a blastocyst with a blastocoel cavity during embryonic development after fertilization.
Step-by-step explanation:
Each egg cell develops in its own fluid-filled space called a follicle. During embryonic development, after fertilization, the resulting zygote undergoes a process of cleavage forming a morula which then transforms into a blastocyst. This blastocyst is characterized by an inner cell mass within a fluid-filled cavity called the blastocoel, surrounded by trophoblasts. As it implants in the uterine wall, the blastocoel plays a vital role in the developing conceptus that will eventually form the embryo and the associated fetal membranes and placenta.