Apparently, it was originally written as a four-parter and had to be cut down to three parts, so some connections got lost. I'd say that essentially it's about life/change/evolution versus sterility/definition/death.
I've never heard this one before. Off the top of my head I can only think of them ever doing that with Planet of Giants. That one was filmed and then trimmed, rather than reworked before shooting. Do you know if this one was the same?