You've got a few spelling and grammar errors throughout nothing atrocious but they were noticeable. Read through your stories aloud a time or two before publishing. It'll help you catch most of the errors by forcing you to slow down while doing the editing. Also only posting the story as a DOC slightly limits your audience. Save it as at least a RTF, TXT, or PDF. These two programs can help with that:
http://sourceforge.net/projects/pdfcreator/ http://calibre-ebook.com/
That bit out of the way the story was interesting if a bit generic. A bit more on the Doctor's motivations or a bit more on the main character before the transformation would have made us connect in some fashion with either character and made for a much stronger story. As I've read this it's good for pure fetish value but if you want to go beyond that need to work on your characters a bit more. Next, halfway through your transformation you got a little lazy and obviously were pushing to finish faster. If you had kept up with a detailed scientific narrative of each change with the mental reactions of the main character this is a much stronger and more detailed and interesting story. To me it feels like you did this in about a sitting and while it's quite good for that it would've been better if you had spent a bit longer on it.
With all those critisicsms out of the way you've got potential and I'd like to see what you come up with next. I really think the thing you should focus on for your next work is your characters. It doesn't need to be long but give us bits on their lives and their relationships to make us feel for them either liking or detesting them. That always makes for a better story. I'm interested to see what you come up with next.