For salmon roe or small scallops, about 3/8 inch of space is needed, and the roll filled with the roe. In the case of sea urchin (uni is an acquired taste/feel, though actually quite bland), the space should be closer to 3/4 inch. Sea urchin is spooned into about half of the remaining space. Usually, the sea urchin is packed slightly and the unbroken yolk of a quail egg (uzura no tamago) is placed on top of the sea urchin. (This is also an alleged aphrodisiac, and not traditionally offered to women.)
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