Slugs are hermaphrodites, having both female and male reproductive organs. Once a slug has located a mate, they encircle each other and sperm is exchanged through their protruded genitalia. A few days later, the slugs lay around thirty eggs in a hole in the ground, or beneath the cover of an object such as a fallen log.
Slug eggs. These eggs are 5- 6 mm in diameter. Developing slugs can be seen within the eggs.
They are probably eggs of the slug Limacus flavus (Yellow garden slug) as several of them were found the same day in the same area.
Click photo to enlarge.