What is a key characteristic of a blastocyst?

The correct answer highlights that a blastocyst is formed during the cleavage stage of embryonic development. After fertilization, the zygote undergoes a series of mitotic divisions known as cleavage, resulting in a multicellular structure. As these divisions continue, the cells increase in number while the overall size of the embryo does not change dramatically. Eventually, this results in the formation of the blastocyst, which is characterized by a hollow structure filled with fluid and an inner cell mass that will develop into the embryo itself.

In the context of the other choices, stating that the blastocyst is the initial stage of the embryo is misleading; the zygote is technically the first stage. Additionally, the blastocyst is not formed after implantation; rather, it must implant into the uterine lining to establish a pregnancy. Lastly, while the blastocyst does produce hormones after implantation, this function is more associated with the placenta and not a defining characteristic of the blastocyst itself. Thus, recognizing that the blastocyst is a product of the cleavage stage accurately reflects its developmental timeline and role in early embryogenesis.