Verses 15-20. See also Mr 7:17-23. Then answered Peter -- declare this parable. See Barnes "Mt 13:3".

The word parable sometimes means a dark or obscure saying, Ps 78:2. Peter meant, Explain to us more fully this obscure and somewhat novel doctrine. To us, now, it is plain: to the disciples, just coming out of Judaism, taught by the Jewish teachers, the doctrine of Jesus was obscure. Mark says that the disciples asked him. There is no contradiction. The question was put by Peter in the name of the disciples; or several of them put the question, though Matthew has mentioned only one. An omission is not a contradiction.