Why did the narrator use the phrase "fit together like puzzle pieces" to describe how the cells of a developing human embryo form a human face?

The narrator uses that phrase because the pieces of the skin and face feathers have to come together as one to create a human head so they would be fitting together like puzzle pieces.

Explanation:

The human face doesn’t form all at once. It requires many divisions of cells. As the cells divide, they eventually piece together to form a human face.

Explanation:

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.

multiple allele inheritance is traits controlled by more than 2 alleles (ex: pigeons having 3 alleles of one gene govern their feather color).   so the answer is a.