In a medium bowl, mix together ricotta cheese and eggs until blended. Stir in spinach, mozzarella cheese, 1/4 cup of the Parmesan cheese, sugar, salt and pepper. Stuff mixture into uncooked pasta shells.
Spread 1/2 cup spaghetti sauce in the bottom of a medium baking dish. Arrange stuffed pasta shells in a single layer over the sauce. Pour the remainder of the sauce over the shells, cover dish, and chill in the refrigerator for 8 hours, or overnight.
Preheat oven to 400 degrees F (200 degrees C). Bake covered for 40 minutes. Sprinkle with remaining Parmesan cheese, and bake for another 15 minutes.
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