4. The Dog with Two Heads.
This is based off of the same concept as the previous experiment, however, rather than attaching an animal to a machine in order to sustain life, it was surgically attached to the body of another dog. This experiment was imperative to the beginning of our understanding of modern day stem cell research. It was actually found that if the head is not rejected from the body, the process of aging will reverse. The older dog did not experience this phenomenon due to tissue rejection. Being arguably one of the most cruel experiments in recent history, we should be grateful at least for what we learned from it.
