Binding of Isaac: Rebirth Wiki

Blastocyst is a boss that can appear:

The large Blastocyst has (except in Repentance) 190 HP / (in Repentance) 300 HP, the medium forms have 75 HP, and the small forms have 30 HP.

Behavior[ | ]

Makes hops towards Isaac and can perform the following attacks:

  • If Isaac is close it, hops in place and upon landing:
    • (except in Repentance) Shoots 8 projectiles in all directions.
    • (in Repentance) Sends out a burst of large disorganized projectiles in all directions.
  • If Isaac is far away it, leaps off-screen and lands on Isaac's position; upon landing:
    • (except in Repentance) Shoots 12 small projectiles in all directions.
    • (in Repentance) Shoots 8 larger projectiles in all directions.
  • Once enough health is depleted, it splits into two medium versions of itself that continue to hop after Isaac. Medium Blastocysts only hop after Isaac if he is within a certain distance from them. Otherwise, they hop around randomly.
    • Each medium form shoots 6 projectiles in all directions by hopping in place. The orientation of the projectiles depends on Isaac's position.
  • Once enough health is depleted, each medium Blastocyst splits into two small versions. Small Blastocysts no longer hop after Isaac and only hop around randomly.
    • Each small form shoots 4 projectiles in the cardinal directions after pausing for a moment.
  • Each of these small Blastocysts leaves an EmbryoEmbryo behind upon defeat.

Blastocyst's large and medium forms deal a whole heart of contact damage.

Gallery[ | ]

Notes[ | ]

  • Blastocyst can be encountered in groups consisting of:
    • 1 large Blastocyst and 2 small Blastocysts.
    • 1 large Blastocyst and 6 Embryos.
    • 2 medium Blastocysts and 2 small Blastocysts.
  • The medium and small Blastocysts projectiles deal full heart damage regardless of chapter.

Trivia[ | ]

  • Blastocyst appeared in the original The Binding of Isaac.
  • An embryo is called a blastocyst when the differentiation of cells within a morula finally begins, usually 4–5 days after the fertilization of an embryo.
  • It's likely that this boss is a reference to the main character of Viviparous Dumpling, one of Edmund Mcmillen's older games.
  • It is very common for the medium form of the boss to spawn as normal enemies in rooms. On even later floors like SheolSheol, the full boss can even spawn as a common enemy.
  • Blastocyst's behavior is very similar to Boss Monstro ingameMonstro's.
  • As Blastocyst divides, its positive facial expressions will become less happy, losing its smile entirely when it becomes an Embryo.
  • If Blastocyst is killed on spikes, none of the smaller versions will spawn. This can also apply if a very large amount of damage is dealt very quickly to any of the phases.
    • (in Rebirth and Afterbirth) This can also happen if Blastocyst is killed while still having a debuff that actively harms it (e.g. poison, fire).
  • Upon killing Blastocyst, it seems as though, for a split second, an audio clip of someone's voice can be heard. Which can be easier to make out with the music turned off.
  • (in Afterbirth † and Repentance) During the Afterbirth+'s release, an unfair room consisting of 8 Blastocysts with a PortalPortal in the middle existed in all environments of Chapter 4 as a result of unintentionally placed full-sized ones rather than small ones. It was acknowledged by Edmund McMillen on Twitter and was fixed shortly after.
  • (in Repentance) Blastocyst's intro animation where it drops in from the ceiling from the original game has been restored and it has new animations for when it divides.

Bugs[ | ]

Bug Bug! (in Repentance) Blastocyst will briefly appear as the largest form before splitting into smaller ones, regardless of which form was killed.