The Social Security Agreement as defined in § 5-154 shall be made retroactive to the extent permitted by the Social Security Act as defined in said section, but not prior to January 1, 1956. The state shall pay for its share of the retroactive Social Security contributions required therefor. Each employee shall be liable for his own share of the retroactive contributions. The share of each member shall be paid from his retirement account. Amounts paid on behalf of a member from his account for retroactive contributions shall be excluded in determining any amounts which may be refundable to such member, or to his beneficiary, in the event of his withdrawal from state service or death.