Complex sentence: There are also several cysts, previously thought to arise from epithelial remanents trapped in embryonic lines of fusion, most of which are now believed to be odontogenic in origin or have an unknown etiology.
Split into simpler sentences: There are also several, previously thought to arise from epithelial remanents trapped in embryonic lines of fusion. Most, of these are now believed to be odontogenic in origin or have an unknown etiology.