The bread may help catch the glass and move it through the intestines. Depending on how large the piece of glass was you will need to watch for an obstruction of the bowel and any damage the glass may cause in the intestines. Watch for bleeding, pale or white gums. If you see this bring the dog in immediately. An obstruction would show sudden lack of appetite, vomiting, diarrhea, and lethargy. This would require an operation.
Obstruction (foreign body)