Blastocyst

Degraded Boss
Small

Medium

Large

**Blastocyst** is a boss that can appear:

* In all environments of Chapter 4 (Womb, Utero, Scarred Womb).
* Only in the Womb and the Scarred Womb.

The large Blastocyst has  190 HP /  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:
	+ Shoots 8 projectiles in all directions.
	+ 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:
	+ Shoots 12 small projectiles in all directions.
	+ 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 Embryo behind upon defeat.

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

## Gallery

* Blastocyst's appearance.
* Medium Blastocyst's appearance.
* Small Blastocyst's appearance.
* Blastocyst as one of Delirium's forms.

## 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 Sheol, the full boss can even spawn as a common enemy.
* Blastocyst's behavior is very similar to Monstro'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.
	+ 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.
* During the Afterbirth+'s release, an unfair room consisting of 8 Blastocysts with a Portal 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.
* 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.