I understand that polite forms like yomimasu are actually (or originated as) infinitive yomi + auxiliary verb masu in its plain non-past form. Then past form yomimashita is just infinitive yomi + plain past mashita.
So I wonder what is the origin of the negative form, such as yomimasen. Given the above, one might naively expect a plain non-past negative form of masu, which would give yomimasanai. Is -masen "explainable" in terms of other forms? Is it a contraction from an originally longer form?