(a) After satisfying the requirements of section 576 of this title, an insurer may invest any of its funds in common shares of stock in solvent United States corporations that qualify as a sound investment, except, that as to life insurers such investments shall further not aggregate an amount in excess of 50 percent of the insurer's surplus over its minimum required surplus.
(b) The insurer shall not invest in or loan upon the security of more than 10 percent of the outstanding common shares of any one such corporation, subject further to amount invested as limited by section 553 of this title. This limitation shall not apply to investment in the securities of any subsidiary corporation of the insurer which is engaged exclusively in a kind of business properly incidental to the insurance business of the insurer.