How do megaspores develop into female gametophytes in Selaginella?

In the genus Selaginella, the development of the female gametophyte, or megagametophyte, is an endosporic process, meaning it occurs largely within the spore wall.12

The process typically unfolds as follows:

• Initiation: Development often begins while the megaspore is still contained within the megasporangium.2
• Nuclear Division: The process starts with free nuclear divisions, where the nucleus divides repeatedly without the formation of cell walls.2
• Cellularization: Eventually, cell walls form, and the developing gametophyte matures into a structure composed of vegetative cells.23
• Rupture and Maturation: As the gametophyte grows, it causes the megaspore wall to crack open, typically in the region of the triradiate ridge. This protrusion usually occurs on the soil after the megaspore has been shed from the parent plant.23
• Final Structure: The mature female gametophyte lacks chlorophyll and relies on nutrients stored in the spore. It develops several archegonia and usually features three groups of rhizoids.3

In some species, this development may be completed while the megaspore is still within the strobilus.3