The transplanted eyes came from tadpole donors genetically modified to generate a red fluorescent protein. As such, the researchers could see under a microscope whether these eyes sent red nerves outward in the body. Half the recipient tadpoles had no such nerves grow, while about a quarter had nerves projecting toward the gut and the other quarter had nerves extending toward their spine.
The six tadpoles that could see well all had nerves plugged into their spine, which makes sense — their eyes apparently linked with their central nervous system.