Fish roe and milt
Roe is the eggs and milt the sperm sacs of the fish, in Hungarian cooking usually from carp. Both cook into tender, creamy pieces in the soup.
Roe and milt are the reproductive organs of the fish: the female's roe is a mass of small eggs in a thin membrane, while the male's milt is a soft, white, convoluted mass. Cooked, roe turns grainy and slightly firm, whereas milt stays tender and almost creamy, like brains or a soft custard, both with a mild fish flavour. Because they are so delicate, they go into a hot but no longer boiling dish near the end, otherwise they fall apart. Fresh ones come from a fishmonger, most easily during carp season from autumn to Christmas; you often have to order them ahead or buy a whole carp with its innards, and Eastern European grocery shops sometimes stock them frozen.
Substitute:
Fresh roe and milt from other freshwater fish such as pike or zander can be used the same way. Salted roe or caviar is no substitute, as it is too salty and does not cook into the same soft pieces. Leaving both out still gives a good dish, but it loses its distinctive creamy addition.