Slice squash to make 6 cups. Cook, covered, in a small amount of salted water until tender. Drain well, sprinkle with salt. Mix together egg yolks, yoghurt, and flour. Fold in egg whites. IN a baking dish layer half the squash and half the egg mixture, and half the cheese. Repeat layers of squash, egg mixture and cheese. Combine butter and crumbs, sprinkle around edge. Bake in 350 degree oven for 20-25 minutes.