Fact: The headcrab model that has been shipped in the final game is actually an older version of the headcrab. The first 2 pictures are the comparison between alpha and retail headcrab, and the last picture is the later headcrab that was shipped in Half-Life Day One.
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While the Day One Headcrab differs, it is unknown to its intention. - Could of been lost in the VSS crash that happened right before the game went gold. - Could of been an edit that they made and decided against. - Could of been made as a way to identify Day One screenshots. - Could of been a layering error in the texture's source files. We would need builds of HL (full game, not Day One) from August of 1998 and September of 1998 build to verify what the Headcrab textures looked like at those points in development. The Baby Headcrab being out of date in the final game can be proven with Half-Life: Source and PlayStation 2 Half-Life, as they both share a chrome texture that differ from the final game.