I found an interesting reference on this question
Excerpt
Section 2 of the 14th Amendment to the Constitution provides, “Representatives shall be apportioned among the several states according to their respective numbers, counting the whole number of persons in each state, excluding Indians not taxed.” Section 1 of the same Amendment provides, “All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside.” The order of these two Sections implies that the persons upon whom apportionment is to be made are persons born or naturalized in the United States.