Not many people know this (though probably many here I'd bet), but gelatin is made from collagen, which comes from animal bones and skin. Looking it up on Google/Wikipedia, it's derived from pork skins, pork, horses, and cattle bones, or split cattle hides. Its composition is 84%-90% protein, 1%-2% mineral salts and 8%-15% water. Gelatin contains no fat and has about 95 calories per ounce. An ounce of gelatin satisfies about 2% of your daily needs for iron and calcium. By itself, it's essentially odorless and tasteless.
Personally, I always assumed that the gelatin discs in "Allegiance" were made from the collagen from the bodies of the previous inhabitants of the prison cell! Brings a whole new darker spin to the episode, huh? What would've happened to them if Picard hadn't discovered that the Starfleet cadet was an imposter and the aliens ended the experiment?