Cook chicken in boiling salted water until tender. Drain, reserving broth.
Remove skin and bones and dice meat finely. Rinse and dry mushrooms.
Remove, finely chop and reserve stems. Sprinkle mushroom caps with lemon juice. Melt butter and add stems and garlic and cook 10 minutes, stirring now and then.
Add 2 to 3 tablespoons reserved chicken broth and parsley. Season to taste with salt and pepper. Add chicken and egg yolk and mix. Spoon into mushroom cavities.
Arrange on lightly greased baking sheet and sprinkle with bread crumbs. Bake at 350 F 15 to 20 minutes. Serve hot.