As the embryo develops in the female gametophyte which is retained on parent  sporophyte.

Selaginella and salvinia are heterosporous as they produce two kinds of spores- micro ,small one and megaspores, big one. The microspores and megaspores germinate and give rise to male and female gametophyte in these plants and retained on the parent sporophyte for variable periods. The development of zygote into young embryos take place within the female gametophyte. This event is precursor to the seed habit considered to an important step in evolution.