http://www.eidolons-inn.net/old/sega/gen_info.html

But one rare variation of the Genesis deserves special mentioning: The Sega Terra Drive. This was an ordinary AMD 386SX PC (25MHz) with 2MB of RAM - but it had a built-in Mega Drive as a 16bit ISA cart. In Japan, it was shipped with a set of development tools which made it possible to program your own Genesis games without further hassle. However, it wasn't very successful in Japan, mainly due to its high price (about $3000). The Terra Drive was sold in Europe by Amstrad with the name "Mega PC".