I feel that the seeds Master Xehanort was referring to are Vanitas, Braig, and Isa. You see, they both had their normal characteristics at first. But one minute later, their eyes and ear changes to Master Xehanort's yellow eyes and pointy ears. Nomura has hinted that this shows a connection to Xehanort and that something even happened between him and Braig that gave him those changes after he fought Terra in Radiant Garden. My theory is that Master Xehanort likely put some fraction of his heart inside them, as hearts are known to change the appearance of characters such as how Roxas looks like Ventus and how Terra-Xehanort had Master Xehanort's yellow eyes when his heart enter Terra's body. He likely did this to increase his chances of survival which is blatantly exactly like how Voldemort crafted the Horcruxes.
Now, since Vanitas was defeated by Ventus, his heart likely went to Xehanort as his fraction would likely pull him towards the original structure. As for Isa and Braig, the same could've happened to them after their Heartless' were defeated as their hearts were released with no bounds except to Xehanort. This could be why they're in the Realm of Sleep instead of getting revived like Lea, Aeleaus, and Ienzo. Credit for the whole Horcrux theory goes to Grass and his wonderful theories, but I created the theory about how Isa and Braig's hearts are in the Realm of Sleep.
Now, since Vanitas was defeated by Ventus, his heart likely went to Xehanort as his fraction would likely pull him towards the original structure. As for Isa and Braig, the same could've happened to them after their Heartless' were defeated as their hearts were released with no bounds except to Xehanort. This could be why they're in the Realm of Sleep instead of getting revived like Lea, Aeleaus, and Ienzo. Credit for the whole Horcrux theory goes to Grass and his wonderful theories, but I created the theory about how Isa and Braig's hearts are in the Realm of Sleep.