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Blastocyst full

Offical artwork by Edmund McMillen

The Womb

Related Boss(es):


  • Split into two medium parts
    • Smaller parts split into small parts
  • 8-way Bullet shot
    • Cardinal bullet shot
    • Diagonal bullet shot

Effective item(s):


The Blastocyst is a rare boss composed of a large gelatinous bubble with an Embryo in its centre that will occasionally appear in the Womb/Utero. Rarely, instead of the large form, two medium and two small Blastocysts will appear. Several non-Boss variations with a shell are occasionally found in the Womb (or very rarely in the Depths), though the weakest versions (with just the non-hostile Embryo) can be found in most floors, particularly in rooms with a blood donation machine. Due to its behavior, it is essentially a weaker Monstro version encountered in The Womb.


In its largest form, Blastocyst can do a low hop, sometimes scattering bullets all over the place. It can also do a large jump shooting in 8 directions upon landing. It will split into two smaller versions upon taking enough damage.

In its next form, Blastocyst can shoot a 4 way shot either in the four cardinal directions or diagonally when it lands after jumping. It also splits into 2 smaller forms upon taking enough damage.

In its next form, it fires a 4 way shot in the four cardinal directions after being still for a short time. It does not shoot when landing after jump. It loses its bubble upon taking enough damage and becomes an Embryo.



Embryos hop around aimlessly, but can still deal contact damage. This enemy can spawn on any floor and is usually met in a room with a Blood Donation Machine. Unlike other common enemies, Embryos have a health meter, which is to make players, who have yet to face Blastocyst, wonder why they do.[1]



  • Also known as "Blobby" or "The Embryo" according to the Unholy Edition art booklet.
  • In mammalian biology, a blastocyst is a hollow fluid-filled cellular structure formed in early pregnancy which contains the cells which will eventually form an embryo.
  • Embryos use the same death sounds as gooballs from the game World of Goo.


The Binding Of Isaac BLASTOCYST00:57

The Binding Of Isaac BLASTOCYST

Basement Larry Jr.MonstroThe Duke of FliesGeminiStevenFamine
Cellar WidowPinThe Duke of FliesFamineGurdy Jr.The Blighted Ovum
The Caves FistulaPeepGurdyChubC.H.A.D.Pestilence
Catacombs The HuskThe HollowThe Blighted OvumThe Carrion QueenPeepPestilenceThe WretchedGurdy Jr.
The Depths LokiMonstro IIGishWarMom
Necropolis Mask of InfamyDaddy Long LegsThe Carrion QueenThe BloatLokiWarThe WretchedMom
The Womb ScolexBlastocystThe BloatDeathConquestDaddy Long LegsTriachnidTeratomaMom's HeartIt Lives
Utero TeratomaThe BloatDeathConquestDaddy Long LegsTriachnidLokiiMom's HeartIt Lives
Sheol The FallenSatan
Cathedral Isaac
The Chest ???
First to fifth and seventh level The FallenHeadless Horseman
Devil Room Krampus

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