Fill in the blank with the best possible answer.

1		The period begins with fertilization and ends at birth.
2		The sperm must digest the in order to reach the egg.
3		Fusion of the sperm and egg nuclei results in a cell with 46 chromosomes called a(n) .
4		Individual cells produced by cleavage are called .
5		The becomes the extraembryonic membranes and the placenta.
6		The hormone that causes uterine contractions is .
7		The hormone inhibits the uterus and weakens joints in the mother.
8		All adult muscle tissue comes from the fetal germ layer the .
9		The germ layer called becomes the nervous system.
10		The produces the stem cells of bone marrow.
11		The becomes the umbilical blood vessels.
12		Some blood from the umbilical vein bypasses the liver through the .
13		An opening in the fetal interatrial septum is called the .
14		The hormone that causes milk production is .
15		The process by which the uterus returns to its prepregnancy size is called .