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 an Embryo inside.

Behavior

• Hops in place and shoots eight bullets in all directions.
• Leaps off-screen and lands on Isaac's position, shooting twelve bullets in all directions upon landing.
• Once enough health is depleted, it will split into two medium versions of itself, which shoot six bullets in all directions.
  • The medium Blastocysts have 75 HP each.
• Each of these two medium Blastocysts split into two small versions, which shoot four bullets in the cardinal directions.
  • Small Blastocysts have 30 HP each.
• Each of these small Blastocysts leave an Embryo behind upon defeat.

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.

Notes

• Blastocysts are seen in a group consisting of either 1 large Blastocyst and 2 small Blastocysts, 1 large Blastocyst and 6 Embryos, or 2 medium Blastocysts and 2 small Blastocysts.

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.

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.