Which of the following is true of the embryonic stage of gestation?
A. Some cells around the edge of the blastocyst cluster to form a thickened cell mass from which the embryo begins to develop.
B. The embryo becomes a fetus in the beginning of this stage.
C. The first bone cells appear during this stage.
D. Defects that occur in this stage are often the most serious and are likely to cause spontaneous abortion.