Question

multiple choice question
what structural changes or events mark the beginning of the embryonic stage of development?
the second polar body is expelled
the morula forms
the blastocyst develops
the inner cell mass forms the embryonic disc

Explanation:

The embryonic stage begins when the inner cell mass forms the embryonic disc. The expulsion of the second polar body occurs during fertilization. The morula is an early stage before the blastocyst. The blastocyst is a stage before the formation of the embryonic disc which is crucial for the start of the embryonic stage as it gives rise to the embryo proper.

Answer:

The inner cell mass forms the embryonic disc