Developing frog egg. Light micrograph of a vertical section through a developing egg laid by a common frog (Rana temporaria temporaria). The egg has reached the yolk plug, gastrula stage. The outer cells have flowed into an invagination (centre right) to form the yolk plug. The outer cells have also formed three layers: an outer ectoderm, a middle mesoderm and a yolky inner layer the endoderm, enclosing the blastocoele (central cavity). Magnification: x100 when printed at 10 centimetres high.