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The Blastocyst is a rare boss composed of a large gelatinous bubble with an Embryo in its centre that will occasionally appear in the Womb/Utero. Rarely, instead of the large form, two medium and two small Blastocysts will appear. Several non-Boss variations with a shell are occasionally found in the Womb (or very rarely in the Depths), though the weakest versions (with just the non-hostile Embryo) can be found in most floors, particularly in rooms with a blood donation machine. Due to its behavior, it is essentially a weaker Monstro version encountered in The Womb.
In its largest form, Blastocyst can do a low hop, sometimes scattering bullets all over the place. It can also do a large jump shooting in 8 directions upon landing. It will split into two smaller versions upon taking enough damage.
In its next form, Blastocyst can shoot a 4 way shot either in the four cardinal directions or diagonally when it lands after jumping. It also splits into 2 smaller forms upon taking enough damage.
In its next form, it fires a 4 way shot in the four cardinal directions after being still for a short time. It does not shoot when landing after jump. It loses its bubble upon taking enough damage and becomes an Embryo.
- Also known as "Blobby" or "The Embryo" according to the Unholy Edition art booklet.
- In mammalian biology, a blastocyst is a hollow fluid-filled cellular structure formed in early pregnancy which contains the cells which will eventually form an embryo.