Blastocyst

Blastocyst is a boss that can appear: The large Blastocyst has 190 HP, the medium forms have 75 HP, and the small forms have 30 HP.
 * In all environments of Chapter 4.
 * Only in and.

Behavior
Makes hops towards Isaac and can perform the following attacks: Blastocyst's large and medium forms deal a whole heart of contact damage.
 * 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.

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 music turned off.
 * During the DLC'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.