Quote: Late response, really hard to tell from the photo, and this might be totally off, but I wonder if that's an organized hematoma (maybe something-or-other ruptured, leading to a bleed and ultimately an infection? Just guessing). They can take on quite a bit of structure and look like "tissue" when they begin to organize if it's been a little while since the bleed. Did you find any red brown staining anywhere else? (Of course, if abdomen was full of pus, you may not have been able to see...) Still don't know a lot about chickens' innards, though...
You could be right, it does look more like it cam from blood. It was also located at the bottom of her abdominal cavity. She was pretty smelly, so I didn't do a very thorough exam and could easily have missed the source of a bleed. Have more pictures, but they aren't appropriate for this thread.
-Kathy