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 blastula stage. The cell cleavages of the egg are not equal so the animal pole (top) has more and smaller cells, whilst the yolk at the vegetative end (bottom) slows down cleavage and results in fewer and larger cells. The vitelline membrane surrounds the egg. Inside the blastula there is a hollow filled with fluid, the blastocoele. Magnification: x100 when printed at 10 centimetres across.