“A floodlight bathed him. The pure white fabric of his lab coat glowed and his arms were stretched wide, like wings. He looked like an angel. Every face that passed he measured and judged. Male and female. Old and young. The man in the glowing lab coat plucked and sifted and pointed them into lines.” 

This is Yael’s initial perception of Dr. Geyer. In the mind of a six-year-old, he appears as an angel based on his physical characteristics. Like nearly all the characters in the novel, Geyer’s appearance is deceiving. He is the Angel of Death. 

“It was this part of his face she always focused on when he spoke. The gap. The not-quite-fullness of his soft words. The single break in his paternal mirage.” 

As little Yael comes to know Geyer and his capacity to cause pain, she begins to understand his true nature. Geyer appears solicitous of her physical health but actually isn’t. Yael keys on the gap in his teeth to represent the gap between appearance and identity. 

“Take all the colors and feelings and human inside. Drain, drain, drain until nothing was left. Just a ghost of a girl. A nothing shell. Progress.”

Yael is describing Geyer’s exultation as he observes her basic physical characteristics slipping away. She is becoming his chemical creation. As he gains more of what he wants from her, she loses what was left of herself.