Contracts issued by a mutual assessment life, accident and sickness insurer shall be on forms prescribed by the insurer and shall be substantially uniform among members of the respective classes of insurance written by the insurer. Each member shall pay his pro rata share of all losses or damages sustained, expenses of operations of the insurer, and the maintenance of an adequate surplus to policyowners as determined by the board of directors. Periodic assessments may be collected as advance premiums, or by past assessments, or by both methods. The amount of assessments shall be established by the board of directors of the insurer. When a contract is subject to assessment, the contingent liability of each member of an insurer shall be clearly stated in the contract. Contracts omitting the right of contingent assessment shall be deemed to be nonassessable.