Of course, a petri dish is not a brain, and the petri dish system lacks certain crucial components, like immune system cells, that appear to contribute to the devastation once Alzheimer’s gets started. But it allows researchers to quickly, cheaply and easily test drugs that might stop the process in the first place. The crucial step, of course, will be to see if drugs that work in this system stop Alzheimer’s in patients.

The discovery, said Dr. Sam Gandy of the Icahn School of Medicine at Mount Sinai in New York, is “a real game changer” and “a paradigm shifter.” He added, “I’m really enthusiastic to take a crack at this in my lab.”

Karen Duff, an Alzheimer’s researcher at Columbia University, while praising the work as “a tour de force,” cautioned that once Alzheimer’s starts, tangles can take off on their own and may need to be attacked by drugs that strike them specifically in order to stop devastation in the brain.

Dr. Tanzi is now starting an ambitious project to test 1,200 drugs on the market and 5,000 experimental ones that have finished the first phase of clinical testing — a project that is impossible with mice, for which each drug test takes a year. With their petri dish system, Dr. Tanzi said, “we can test hundreds of thousands of drugs in a matter of months.”

He already has used his system to look at drugs designed to prevent the formation of amyloid, the protein that clumps into plaques. The drugs, he reports, prevented both plaques and tangles in the petri dishes. Some are in clinical trials, and it is not known if they work in people. One was tested in patients and failed because it was too toxic. One hope is to find drugs for other diseases that are known to be safe and work on Alzheimer’s in the petri dish.