A series of experiments that have been made on the eggs of sea-urchins has led to equally important results. The earliest experiments are those of O. and R. Hertwig, who, in addition to studying the effect of different drugs on the developing egg, found that fragments of the eggs of sea-urchins, obtained by violently shaking the eggs in a small vial, could give rise, if they contained a nucleus, to small whole embryos. Boveri made the important discovery in 1889 that if a non-nucleated piece of the egg of the sea-urchin is entered by a single spermatozoon, the piece develops into a whole embryo of a size corresponding to that of the piece. Fiedler, in 1891, separated the first two blastomeres by means of a knife, and found that the isolated blastomere divides as a half, but he did not succeed in obtaining embryos from the halves. Driesch has made many experiments, beginning in 1891, with the eggs and embryos of the sea-urchin. He separated the first two blastomeres (’91) by means of Hertwig’s method of shaking the eggs, and studied the development of the isolated blastomeres. He found that the cleavage was strictly that of a half, and not like that of a whole egg. The normal egg divides into two, four, and eight equal parts. At the next division, four of the cells divide very unequally, producing four very small cells, the micromeres, at one pole. The four cells of the other hemisphere divide equally (Fig. 64, _I_). The isolated blastomere divides at first into two equal parts, then again into equal parts. At the next division two of the cells produce micromeres and two divide equally (Fig. 64, _G_). This is exactly what happens at this division in each half, if the blastomeres are not separated. In later stages a half-sphere is formed that is equivalent to half of the normal sphere (Fig. 64, _C_). The open side corresponds to the side at which the half would have been united to the other half. Thus up to this point a half-cleavage and a half-blastula have appeared.[114] In later stages the open half-blastulæ close in, producing a whole sphere that becomes perfectly symmetrical (Fig. 64, _D_). A symmetrical gastrula develops (Fig. 64, _E_) by the invagination of a tube at one pole, and a symmetrical embryo is formed (Fig. 64, _F_) that resembles the normal embryo except in point of size. Driesch has also found that a number of twin embryos arise from the shaken eggs. They arise from eggs whose blastomeres have been disturbed or shifted, so that each produces a small whole embryo, the two embryos being united to each other in various ways.