Blastocysts has four forms, Large, Medium, Small and Embryo. Their embryo forms can be found throughout all floors of the game in Blood Donation Machine Rooms, even though they can't attack they have a health bar like a regualr boss. In later floors beyond the Womb you can find two medium ones in rooms like many bosses in the late game. The large version is found rarely in the Boss Room of Womb part 1, when defeated he will drop a common boss drop (if you have WTOL he can also drop a common boss drop from it).
Blastocysts fights like Monstro with him hoping around, in his large form he has a small jump which fires bullets randomly and a big jump that fires a 8 directional shot on his landing. After enough damage he will split into two medium Blastocysts, like his earlier form he shoots bullets after jumping but he will either shoot 4 bullets in a X or 4 in + shape. Like before after enough damage the medium Blastocysts will split int two small Blastocysts, the small ones will shoot 4 bullets in a X after stayin still for a few seconds. After enough damage the small Blstocysts bubble will pop leaving them as a Embryo which can only flail on the ground.
