The Reproductive System — John Sladek

Wompler's Walking Babies once put Millford, Utah, on the map. But they aren't selling like they used to. In fact, they aren't selling at all and the only alternative to winding the company up is to tap the government for a research grant. And so Wompler Research Laboratories and Project 32 come into being. The plan is to produce self-replicating mechanisms; identical cells equipped to repair intracellular breakdowns, convert power from their environment and create new cells. But suddenly the nondescript grey metal boxes start crawling about the laboratory, feeding voraciously on any metal... and multiplying at an alarming rate.