When one joint tenant dies before the severance, his interest does not survive to the other joint tenants but descends and vests as if his interest had been severed and ascertained; provided, that in the event it is stated in the instrument creating such tenancy that such tenancy is with right of survivorship or other words used therein showing such intention, then, upon the death of one joint tenant, his interest shall pass to the surviving joint tenant or tenants according to the intent of such instrument. This shall include those instruments of conveyance in which the grantor conveys to himself and one or more other persons and in which instruments it clearly appears that the intent is to create such a survivorship between joint tenants as is herein contemplated.