Dissolve yeast in warm water and add to lukewarm milk. Add butter, lemon rind, sugar, and almond extract and mix well. Add flour and eggs. Knead until smooth and allow to rise until doubled in bulk. Then knead again. Divide dough into three parts. Roll each portion into a long strip. Braid three strips into roll. Place on buttered pan, cover with towel and allow to rise until doubled in bulk again. Brush with beaten egg yolk. Sprinkle top with thin slivers of almonds and bake in moderate oven for about 30 minutes.