The research tree in this game had a few bugs which prevent you from researching certain items, one of them can even stop you from successfully completing the game.
Luckily there are guides that contain ways to avoid these bugs.
"X-Com II" is heavily based on the writings of Howard Phillips Lovecraft.
Many names and creatures are taken straight from HPL. Also, T'leth is R'lyeh,
and in the final mission, you can see the tomb and the portrait of the Great
TFTD added hand-to-hand weapons to the usual arsenal of weapons (unless you count the stun rod as a hand-to-hand weapon).
"After completing UFO MicroProse wanted to do a quick follow up within six months. We said that this was not feasible, and if it were possible it would be little more than the same game with different graphics. Instead we started work on X-Com:Apocalypse, which was much more ambitious. Once UFO/X-Com was clearly known to be a success, MicroProse suggested that we license the code for them to develop their own sequel. The rest is history."
-- taken from the Mythos Games web site.