"An additional parol agreement omitted from the executed writing, existing prior to or contemporaneous with that embodied in the writing, may not, under the guise of proving consideration, be added to the writing itself." See, also, In re Nielsen's Estate, 272 Mich. 636. Having determined that there was no competent testimony offered to vary the terms of the deed of November 1, 1919, we now consider the nature of the deed between father and son. Defendant claims this deed created a joint tenancy in the father and son.