Primroses are unusual in having two almost identical, yet slightly different types of flowers.
In the thrum-eyed type (seen on the left), the style reaches only halfway up the flower tube, so that the stigma is also positioned halfway up inside the tube. The anthers in this type are at the top of the flower tube and can be seen as a mass in the centre of the flower.
In pin-eyed flowers (on the right), the stigma is at the top of the flower tube and can be seen in the centre of the flower looking like a small green pin head. In this type of flower, the anthers are halfway down the central flower tube, in a ring around the style.

Fertilisation is usually achieved between a thrum-eye flower and a pin-eye flower; pollination from pin to pin or thrum to thrum is not generally effective.

Next time you see primroses, or any other member of the primrose family, take a closer look.