Changes in shape sometimes occur during the growth of the pearl, the tendency being always toward the rounding of the surface. If the nucleus is fast to the shell, a dome is built over and around it. If the nucleus permits, the nacre is deposited not only over but under its edges to the point of contact with the shell, so that a button pearl connected with the shell at the centre only, results. Two pearls held against the shell and growing side by side are separately enveloped until they touch each other, after which they are included in single deposits of nacre and the depression between their domes becomes less distinct with each successive coating. Similarly, a cluster of small pearls lying together often forms the nucleus of a large rounded baroque or button pearl. Examination of such formations shows, that up to a certain period the pearls have a separate existence and growth. They then become joined in an irregular mass of twinned pearls, and finally, if allowed to remain in the oyster long enough, all individuality is lost in the tendency to round over. The same thing occurs when grains of sand or other intrusions become attached to a growing pearl. They are quite prominent when first included in the nacreous deposit and can be easily detached from the under pearl by breaking through the layer which binds them on; but they are soon obliterated by succeeding deposits. This filling-in process is sometimes accomplished by additional layers in the depression, sometimes by thicker layers. It happens occasionally, when skinning a round pearl, that one of these fillings is uncovered and flakes out, leaving the pearl irregular, as it was in a former stage of its growth. Although pearls naturally grow spherically, many free pearls are more or less buttoned, that is, have a flat place from which the pearl rises like a dome, high or low. This happens when the pearl is held during growth by the fish against the shell with a part of its body intervening. According to circumstances, the pearl varies in form from slightly button, to a low dome, rising from a plane at its greatest diameter. Should a pearl of this description become dislodged, the rounding action of the mollusk would begin at once to obliterate the plane.