The Thing (spoilers)

[Big Orange]

When I said "Spider-Human Monster" I meant that thing that explodes out of the guy's chest and attachs to the ceiling after it kills the Doctor, not the Headcrab. Where did it get the Mass to make THAT out of that guys' body when the body is still lying there?

And where did Blair get the Mass to become that huge worm-dog-dinosaur monster thing at the end?

We don't know exactly where the Thing organisms start and where they end; each Thing cell seems to be a entity in itself and Thing conglomerates may expand through cell replication, with base materials extracted from everything in contact (ie water, carbon, iron etc) to become a huge purple mass of piss and hatred.