controlled group of corporations
For purposes of this subpart, all employees of all corporations which are members of the same controlled group of corporations shall be treated as employed by a single employer. In any such case, the credit (if any) determined under section 51(a) with respect to each such member shall be its proportionate share of the wages giving rise to such credit. For purposes of this subsection, the term “controlled group of corporations” has the meaning given to such term by section 1563(a), except that—