This image originates in the "Book of the Holy Trinity and Description of the Secret of the Transmutation of Metals" from 1420.
Like all Rebis images, the figure is comprised of a male and female half as it is the result of bringing together opposites and resolving them into a single, unified whole. These two halves are commonly demonstrated with strong gender imagery not just in alchemy but also wider occult systems and even many ancient cultural outlooks in general.
King and Queen:
Lead and Gold:
Lead is also seen as passive, feminine and material, the "fixed principle." Gold is active, male and spiritual, the "volatile principle." These are, again, opposite natures that need to be unified for one to achieve perfections.
The Egg and the Basilisk:
Eggs are a symbol of regeneration, potential, development, creation and often the uniting of male and female, which is biologically necessary to produced a fertilized egg.
A piece of pear-shaped piece of lab equipment was also sometimes called the Philosopher's Egg. It was used for condensing materials after distillation, helping to transform materials into pure components.
The basilisk emerging from the egg represents the early stages of the process of uniting divisions within the self. Mythologically speaking, the basilisk was sometimes thought to breathe fire as a dragon does (another common alchemical symbol), although they were more often known for turning people to stone with its gaze like Medusa. Fire is symbolically transformative, and the basilisk is associated specifically with the transmutation of metals. Here it is connected physically with the stones of gold and lead.