Lat take a cat, and fostre him wel with milk,
And tendre flesh, and make his couche of silk,
And lat him seen a mous go by the wal;
Anon he weyveth milk, and flesh, and al,
And every deyntee that is in that hous,
Swich appetyt hath he to ete a mous.

And here's a run at a modern version:

Let (us) take a cat, and foster him well with milk,
And tender flesh, and make his couch of silk,
And let him see a mouse go by the wall;
Soon he waives (relinquishes) milk, and flesh, and all,
And every dainty that is in that house,
Such appetite has he to eat a mouse.