We will consider the formation of the dorsal wall first, and that of the ventral wall afterwards (Figures 1.143 to 1.147). In the middle of the dorsal surface of the embryo there is originally, as we already know, the medullary (mr) tube directly underneath the horn-plate (h), from the middle part of which it has been developed. Later, however, the provertebral plates (uw) grow over from the right and left between these originally connected parts (Figures 1.145 and 1.146). The upper and inner edges of the two provertebral plates push between the horn-plate and medullary tube, force them away from each other, and finally join between them in a seam that corresponds to the middle line of the back. The coalescence of these two dorsal plates and the closing in the middle of the dorsal wall take place in the same way as the medullary tube, which is henceforth enclosed by the vertebral tube. Thus is formed the dorsal wall, and the medullary tube takes up a position inside the body. In the same way the provertebral mass grows afterwards round the chorda, and forms the vertebral column. Below this the inner and outer edge of the provertebral plate splits on each side into two horizontal plates, of which the upper pushes between the chorda and medullary tube, and the lower between the chorda and gastric tube. As the plates meet from both sides above and below the chorda, they completely enclose it, and so form the tubular, outer chord-sheath, the sheath from which the vertebral column is formed (perichorda, Figure 1.137 C, s; Figures 1.145 uwh, 1.146). (FIGURES 1.143 TO 1.146. Transverse sections of embryos (of chicks). Figure 1.143 of the second, Figure 1.144 of the third, Figure 1.145 of the fourth, and Figure 1.146 of the fifth day of incubation. Figures 1.143 to 1.145 from Kolliker, magnified about 100 times; Figure 1.146 from Remak, magnified about twenty times. h horn-plate, mr medullary tube, ung prorenal duct, un prorenal vesicles, hp skin-fibre layer, m = mu = mp muscle-plate, uw provertebral plate (wh cutaneous rudiment of the body of the vertebra, wb of the arch of the vertebra, wq the rib or transverse continuation), uwh provertebral cavity, ch axial rod or chorda, sh chorda-sheath, bh ventral wall, g hind and v fore root of the spinal nerves, a = af = am amniotic fold, p body-cavity or coeloma, df gut-fibre layer, ao primitive aortas, sa secondary aorta, vc cardinal veins, d = dd gut-gland layer, dr gastric groove. In Figure 1.143 the larger part of the right half, in Figure 1.144 the larger part of the left half, of the section is omitted. Of the yelk-sac or remainder of the embryonic vesicle only a small piece of the wall is indicated below.)