Who is Jacques from The Masterpiece and what is their importance?

Jacques is Claude and Christine's son. He is a frail child with a noticeably large head. His parents neglect him, and Christine, his mother, is without maternal instinct..... pushing her small child away for her husband. Because of their oblivion to their son's needs, Claude and Christine don't even notice the severity of his illness. Claude dies one night in his sleep. After his death, Claude's painting of him becomes the ignoble picture of the Dead Child.