Note I — Chapter III · REPRODUCTION AND DEVELOPMENT.
Reading edition
In some forms of life the opening of the cup marks the position of the future mouth: in others, of the future vent. In yet others it elongates into a slit, occupying the whole length of the embryo; the middle part of the slit closes up, and the opening at the far ends mark the position, the one of the future mouth, the other of the future vent.