Figure 2A. A sagittal section of an embryo of the stage shown in figure 2. x 43. Figure 2B. A transverse section through the headfold of an embryo of the stage shown in figure 2. x 43. Figure 3. A dorsal view of an embryo with about fifteen pairs of somites. x 20. Figures 3A-3D. A series of transverse sections through an embryo of the stage of the one shown in figure 3. x 43. Figure 4. A surface view of an embryo with about twenty pairs of somites. x (about) 15. Figures 4A-4D. A series of transverse sections through the anterior end of an embryo of the approximate age of the one shown in figure 4. x 20. Figures 4E and 4F. Two transverse sections through the thyroid gland of this stage; more highly magnified. x 102. Figures 4G-4M. A series of transverse sections caudad to the preceding. Figure 4H, x 43; other figures, x 20. Figure 5. A surface view, in profile, of an embryo at the time of the origin of the limbs. x (about) 5. Figures 5A-5I. A series of transverse sections through an embryo of the age shown in figure 5. x 7. Figure 5J. A composite drawing of reconstructions of the enterons of two embryos of the age of the one shown in figure 5. One reconstruction was in wax, from sagittal sections, the other was a plotted reconstruction from transverse sections. x 14. Figure 6. A surface view, in profile, of an embryo with well developed manus and pes. x (about) 5. Figure 6A. A reconstruction, plotted from transverse sections, of the enteron of an embryo of about the age of the one shown in figure 6. x 14. Figures 6B-6H. Part of a series of transverse sections from which the preceding reconstruction was made. x 7. Figure 7. A reconstruction of the enteron of an embryo of 42 mm. crown-rump length. Figures 7A-7G. A part of the series of transverse sections from which the preceding reconstruction was made. x 7. Figure 7H. A high power drawing of a portion of the wall of the oesophagus in the region of figure 7A. x 190. Figure 7I. A high power drawing of a portion of the wall of the stomach in the region of figure 7C. Figure 7J. A high power drawing of a portion of the wall of the duodenum in the region of figure 7D. Figure 8. An outline drawing, from the ventral aspect, of the enteron of an embryo of 20 cm. length, at about the time of hatching; made from a dissection. x 1. LETTERING FOR ALL FIGURES