These are fertilized by sperm produced by the male gametophyte developing from the microspore. A tiny gametophyte develops inside the wall of a microspore, producing a pollen grain. Each microsporangium contains diploid microspore mother cells, or microsporocytes. The plants are accompanied by microspore massulae and megasporangia. The microspore has three different types of wall layers. A pollen grain develops from a microspore in the microsporangium and contains the male gametophyte. Under certain stressors such as heat or starvation, plants select for microspore embryogenesis. The smaller of these, the microspore, is male and the larger megaspore is female. Each microspore then divides mitotically to form an immature microgametophyte called a pollen grain. The microspore forms a male gametophyte with a single antheridium which produces eight swimming sperm.