Melt a little butter in a saucepan and stir in half the quantity of flour. When well blended, add the hot stock gradually, stirring well. Then add the vinegar, salt, pepper and the caraway seed. Peel the marrows, cut in 2 inch lengths, sprinkle with salt and let them stand for ½ an hour. Then put them in the sauce, bring to the boil, and simmer till the marrow is quite tender.