...When their report of the first successful fertilization of a human egg in vitro was published in Science, scientists were skeptical. ... Maybe, critics said, the observed cleavage was just a laboratory artifact, something that can happen to unfertilized eggs in a petri dish -- a kind of reflex cell division that will never lead to a healthy, normal child.
'arte' from Greek meaning, art, and 'fact' from Greek meaning, make. Artefact, also spelled artifact, means an object that's not created by nature; it's human made, or crafted by human intervention. Note, the paragraph you provided gives us a really big clue about the meaning of 'laboratory artifact':
". . . something that can happen to unfertilized eggs in a petri dish -- a kind of reflex cell division that will never lead to a healthy, normal child."
Both fertilized and unfertilized eggs divide. The former by sperm, the latter by themselves in a petri dish. Human intervention: placing the unfertilzied eggs into a petri dish thereby altering their environment, and causing the cells to divide, which mimics the process a fertilized egg undergoes.