Blastocyst

Boss Degraded Boss Small Medium Large
Boss

Blastocyst is a boss that spawns in all environments of Chapter 4. It is a large blob with a small Embryo inside.

Behavior

• Hops around the room randomly and shoots 8 tears in all directions.
• Performs a giant leap to the player's position, shooting 12 tears in all directions upon landing.
• Once enough health is depleted, it will split into two medium versions of itself, which shoots 6 tears in all directions.
  • The medium blastocysts have 75 HP each.
• These two medium blastocysts each split into two smaller forms, which shoot 4 tears in the cardinal directions.
  • Small blastocysts have 30 HP each.
• When all the parts of the blastocyst are defeated, they will become a bouncing Embryo that does contact damage.

Blastocyst is perhaps the least complicated boss in this chapter, having essentially one attack that gets weaker for each split. Any sort of piercing tears or items such as Technology or Brimstone can defeat this boss extremely quickly.

Gallery

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  Blastocyst ingame.

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  Medium Blastocyst.

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  Small Blastocyst

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  Blastocyst as one of Delirium's forms.

Notes

• Blastocysts are seen in a group consisting either of 1 large Blastocyst and 2 small Blastocysts, 1 large Blastocyst, and 4 Embryos or 2 medium Blastocysts and 2 small Blastocysts.

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 is very common for the second level of the boss to spawn as normal enemies in rooms. On even later floors like Sheol, the full boss can even spawn as a common enemy.
• 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 with very quickly to any of the phases.
  • This can also happen if Blastocyst is killed while still having a debuff that actively harms it (e.g. poison, fire).
• Upon killing a blastocyst, it seems as though, for a split second, an audio clip of someone's voice can be heard. To better hear it, listen with headphones without the music.
• During the DLC's release, an unfair room with a Portal in the middle existed in all environments of Chapter 4 consisting of 8 Blastocysts as a result of unintentionally placing full-sized ones rather than small ones. It was acknowledged by Edmund McMillen on Twitter and was fixed shortly after.
• As Blastocyst divides, its positive facial expressions will become less happy until it becomes a bouncing embryo which has no smile at all.