Sentence:  Under the law as it existed until 1943 , the Federal Government made grants to the States on the basis of <WSD>population</WSD> , matching State expenditures on a 50 - 50 basis . 
Meaning of the word marked with <WSD> tags:  the number of inhabitants (either the total number or the number of a particular race or class) in a given place (country or city etc.)