From Wikimedia.
Slashed sleeves are fading, but Mary's sleeves are probably made separately from the bodice with the pouffy chemise beneath. Two clasps can be seen attaching the sleeve and more fasten her bodice. She wears a proto-hurluberlu coiffure. Her stays could be what show through the split of her bodice.
Keywords: 1647, van Honthorst, Princess Royal Mary of Great Britain, Mary Stuart, Maria Henriëtte Stuart, Prinses van Oranje-Nassau, Stuart family, Oranje family, British, Princess Royal, Princess, Dutch, proto-hurluberlu coiffure, necklace, veil, chemise, modesty piece, elbow length under-sleeves, turned back cuffs, split bodice, off shoulder scoop neckline, elbow length puffed over-sleeves, clasps, vee waistline, close skirt