This is explained in no 17 of Igusa's paper "[On the theory of algebraic correspondences...](https://projecteuclid.org/euclid.jmsj/1261735488)", J. Math. Soc. Japan 1 (1949), [DOI: 10.2969/jmsj/00120147](https://doi.org/10.2969/jmsj/00120147); it corresponds to Felipe Voloch's guess.

(There are many typos in Igusa's paper, and the Project Euclid scan does not improve them. Better read a hard copy if possible.)