Following the gestation period , a single rubbery-skinned egg between 13 and 17 millimetres in diameter is laid directly into a small , backward-facing pouch that has developed on her abdomen . Following the gestation period , a single rubbery-skinned egg between 13 a in diameter and 1.5 a in weight is laid from her cloaca directly into a small , backward-facing pouch that has developed on her abdomen .