If the ligature is drawn tighter at a later stage, when, for instance, the medullary plate is plainly visible but is still wide open, a different result is obtained. The posterior part no longer forms a new head at its anterior end, but develops into those structures that it would form normally. In some cases it was found that the region from which the ear develops had been pinched in two, and in consequence a small vesicle appears in front of the constriction and another behind it. In those cases in which the ligature lies in the median plane of the embryo, it is found that a double anterior end is produced. As the embryo develops it tends to elongate, and in consequence the material is pushed forward on each side of the ligature. A double head is the result. The extent of the doubling depends on the depth of the constriction between the halves. In the most extreme cases two complete heads are formed with an inner nasal pit, eye, and ear on each head, as well as the normal outer ones. The results show that even such complicated structures as the eyes and ears, etc., may arise from parts of the body where they never appear under normal conditions. [Illustration: FIG. 64.--Sea-urchin egg and embryo. _A._ Two-cell stage. _B._ Same, with blastomeres separated. _G._ Two half-sixteen-cell stages. _C._ Open half-blastula stages. _D._ One of last, later stage, closed blastula of half size. _E._ Gastrula of half size. _F._ Whole pluteus of half size. _H._ A half-sixteen cell dividing in same way as a whole egg (eight cell). _I._ Whole egg at sixteen-cell stage.]