“The Adaptive Ultimate” is a science fiction short story about an experimental medical treatment gone awry. It was written by Stanley G. Weinbaum and first published in the November 1935 issue of Astounding magazine under the pen name “John Jessel”.
In the 1971 Astounding/Analog All-Time Poll, the story tied for 16th in the Pre-1940 Short Fiction classification.
A very unsettling read. This is pretty much a horror story, with a science fiction explanation to give it some degree of plausiblity.