In the Earth-Two timeline, the ability of the intact organ of the brain to be transplanted between different individuals or even different species is feasible.  The procedure is doubtlessly highly complex and has been accomplished through several groundbreaking advancements in science and medicine. Donor brains are taken from individuals who are recently of imminent deceased and survival or retrograde to original donors is uncommon if even possible. 

Among the barriers overcome has been elimination of problems such as immune rejection via tissue or blood type or the ability to modify metabolism to slow sufficiently to keep the recipient subject alive until the full surgery is complete.  The recipient’s brain is removed and the spinal cord, cranial nerves and blood vessels carefully identified for reattachment as the donor brain is implanted. Vascular connections are created first to allow oxygenation and reduce risk of cerebral ischemia and brain death.

The most painstaking process is connection of the two spinal cords.  Composed of millions of nerves, it is unlikely that this achieved by simple surgery but more like by surgical anchors that held the connection combined with neuroregenerative compounds and growth factors that could accelerate the biological re-fusion.  The overall success of the surgery and the time to recovery likely depends on this step.  Finally, the 24 cranial nerves must be reconnected, likely a surgical process that connects 12 pairs of nerves that largely serve the cranium and the vagus nerve, which serves the heart and digestive tract.

Recovery is likely time-consuming and requires prolonged medical sedation. Additional neuroregenerative processes are likely needed combined with neuro=stimulation to accelerate healing and reduce the risk of paralysis. Immunomodulatory process can also be used to eliminate the risk of rejection.

The ethical considerations of brain transplant are considerable.  It goes without saying that taking the body of an unwilling host would be considered highly unethical, though several examples exist on Earth-Two where such prohibitions are ignored.

There are two prominent examples of brain-transplants on Earth-Two, one good and one evil.

The evil one, and the earliest known, is the criminal known as the Ultra-Humanite.  First encountered in 1939, the true origins of his transplants are unknown.  It is known occupied the body an elderly cripple, a movie starlet, a giant insect and a mutated albino ape.  Given the frequency and variety of his transplants, it is likely that the Ultra-Humanite is altered somehow, either through magic, mutation or experimental process, so that is brain itself is more readily and diversely transplanted.

The more noble example is the scientist Robert Crane.  An engineer working on a way to preserve the brains of terminally injured individuals.  Criminals thinking to profit from his work broke in on Crane and his assistant Chuck Grayson and finding nothing of easy value, fatally shot Crane.   Grayson used the methods they two had developed to transplant Grayson’s brain into a robot shell.  The successful operation left Crane in the robot body for a decade, during which time he fought crime ass Robotman.  Caught in a mine shaft that put him in suspended animation for 20 years, Crane’s brain was eventually transplanted into Grayson body when his former assistant had cryo-preserved after being diagnosed with terminal brain cancer.