Preheat the oven to 400 degrees Fahrenheit.

In a mixing bowl, combine ricotta, mozzarella, parmesan, sugar, egg, nutmeg, parsley, salt, and pepper.

Fill each uncooked manicotti shell with the cheese mixture, using a spoon or a piping bag.

Place the filled manicotti in a single layer in a 9x13 inch baking dish.

Heat the marinara sauce until it reaches a simmer, then pour it over the manicotti, making sure they are completely covered.

Cover the dish with aluminum foil, sealing the edges to keep the steam in.

Bake in the preheated oven for 40 minutes.

Remove the foil, sprinkle additional parmesan cheese over the top, and bake uncovered for an additional 5 minutes, or until the cheese is melted and bubbly.