As the portabella mushroom begins to mature having reached its maximum stem height, the veil parts as the cap spreads out and flattens and the gills begin to open in preparation for the dropping of the mushroom spores (like seeds).
A portabella mushroom with a broken veil and still rounded cap
A portabella mushroom beginning to mature will show a parted (broken) veil and yet still have the thick and rounded edges of the mushroom cap common to the immature portabella mushroom. The cap will still be thick and quite 'meaty' and heavy with water weight.