Lycophya-Selaginella-Repro-3

Each Microspore produces one Microgametophyte.

Each Microgametophyte produces one Antheridium.

The Microgametophyte is essentially an Antheridium within the Spore Wall.

The Endosporic Development of the Antheridium means that it is protected by the Spore Wall plus its own Jacket until it is mature.

This helps insure that viable Sperm are produced by each Antheridium.

The Sperm are Biflagellate like those of Lycopodium.

Meiosis produces four Megaspores in each Megasporangium.

Each Megaspore is a Large Cell inside a thick, layered wall.

The Megaspore cell contains a lot of Storage Lipids.

It has a Triradiate Ridge which has three crests or peaks.

Cell Divisions start beneath the crests & nowhere else.  These lead to the formation of a relatively thin cellular pad that lies on top of the storage lipids. Only a small fraction of the Megaspore becomes cellular.
Archegonia differentiate in the cellular pad, just beneath the crests of the Spore Wall.

This reminds me of embryo development in chicks. The embryo starts out as a small disk of cells that floats on the lipid-rich yolk.

Whole Megaspore:Note the Triradiate Ridge

Sections through a megagametophyte



Archegonia formed in the cellular region of the Megagametophyte


Megagametophyte with Archegonia