18 Hans Spemann and Hilde Mangold cells is intercalated in its floor, in sharp contrast to the adjacent regions. This white strip is part of the cristatusimplant that was clearly recognizable from the outside in the living embryo before the neural folds closed(Fig 3). The anterior end of this strip is approximately at the point where the thickness of the neural tube decreases ratherabruptly; it opens to the outside shortly thereafter The strip is wedge shaped, with the pointed edge toward the outside; as a result, only the tapering ends of the cells reach the surface of the embryo(figs. 5 and 6)or the central canal at the short stretch where they border it Fig. 5. Um 8b. Crass sec- tion through middle third of the embryo (df Figs. 2 and 3). pr. Med., pI anday neural tube. The E 3 pr Med implant (light)isin the sec- At its posterior end, the cristatus strip reaches the blastopore, and it is continuous with a mass of cristatuscells that is located between the secondary neural tube and the mesoderm on one side, and the endoderm on the other(Fig. 6). Because of their position one would be inclined to consider these cells as endoderm; but in size they resemble more the mesoderm of the taeniatusembryo, with which they are associated. At any rate, this cell mass, which extends a bit farther rostrad, has reached its position by invagination around the blastoporal lip. There is yet another mass of cristatus cells still farther rostrad. It has the form of a thin plate underlying the anterior part of the induced neural tube, as far s it is closed; at its anterior end and at its sides, it coincides approximately with the edge of the tube. and at its posterior end, it extends to the ectodermal strip of the implant. This plate is incorporated in the normal taeniatus mesoderm(Fig. 4). It is not differentiated further into notochord or somites Fig 6 Um 8b. Cross sectionin the region ofthe blastopore(Bl )(d Figs. 2 and 3). pr: Med, primary meuraltube sec Med. secondary neura/tube. The implant (light) has severalcells in the secondary neural tube, with its main mass in the mesoderm (sec. Mes. crist ).100r Altogether, a rather substantial part of the implant remained in the ectoderm. This portion was greatly stretched in length; as a result, the circular white disk that was implanted has become a long narrow strip that turns inwards around the blastoporal lip. Shifting of cells in the surrounding epidermis may have played a role in these form changes; the extent to which this occurs would have to be tested by implantation of a marker of indifferent material. a piece from a region near the upper lip of the blastopore could handily be considered as suitable for this purpose. We know from earlier experiments(Spemann 1918, 1921)that convergence and stretching of the cell material occurs at the posterior part of the neural plate. It is improbable that the cells of the neural plate are entirely passive in this process; rather, they may have aninherent tendency to shift that perhaps has been, togetherwith other characteristics, induced by the underlying endo-mesoderm. This tendency would be retained by18 Hans Spemann and Hilde Mangold Fig. 6. Um 8b.Cross section in the region of the blastopore (Bl.) (cf. Figs. 2 and 3). pr. Med., primary neural tube; sec. Med., secondary neural tube. The implant (light) has several cells in the secondary neural tube, with its main mass in the mesoderm (sec. Mes. crist.). 100X. At its posterior end, the cristatus strip reaches the blastopore, and it is continuous with a mass of cristatus cells that is located between the secondary neural tube and the mesoderm on one side, and the endoderm on the other (Fig. 6). Because of their position one would be inclined to consider these cells as endoderm; but in size they resemble more the mesoderm of the taeniatus embryo, with which they are associated. At any rate, this cell mass, which extends a bit farther rostrad, has reached its position by invagination around the blastoporal lip. There is yet another mass of cristatus cells still farther rostrad. It has the form of a thin plate underlying the anterior part of the induced neural tube, as far as it is closed; at its anterior end and at its sides, it coincides approximately with the edge of the tube, and at its posterior end, it extends to the ectodermal strip of the implant. This plate is incorporated in the normal taeniatus mesoderm (Fig. 4). It is not differentiated further into notochord or somites. Altogether, a rather substantial part of the implant remained in the ectoderm. This portion was greatly stretched in length; as a result, the circular white disk that was implanted has become a long narrow strip that turns inwards around the blastoporal lip. Shifting of cells in the surrounding epidermis may have played a role in these form changes; the extent to which this occurs would have to be tested by implantation of a marker of indifferent material. A piece from a region near the upper lip of the blastopore could handily be considered as suitable for this purpose. We know from earlier experiments (Spemann 1918, 1921) that convergence and stretching of the cell material occurs at the posterior part of the neural plate. It is improbable that the cells of the neural plate are entirely passive in this process; rather, they may have an inherent tendency to shift that perhaps has been, together with other characteristics, induced by the underlying endo-mesoderm. This tendency would be retained by Fig. 5. Um 8b. Cross sec￾tion through middle third of the embryo (cf. Figs. 2 and 3). pr. Med., primary neural tube; sec. Med., sec￾ondary neural tube. The implant (light) is in the sec￾ondary neural tube. cells is intercalated in its floor, in sharp contrast to the adjacent regions. This white strip is part of the cristatus implant that was clearly recognizable from the outside in the living embryo before the neural folds closed (Fig. 3). The anterior end of this strip is approximately at the point where the thickness of the neural tube decreases rather abruptly; it opens to the outside shortly thereafter. The strip is wedge￾shaped, with the pointed edge toward the outside; as a result, only the tapering ends of the cells reach the surface of the embryo (Figs. 5 and 6) or the central canal at the short stretch where they border it