Blastocyst

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+ expansion'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.