(FIGURE 1.130. Germinal area or germinal disk of the rabbit, with sole-shaped embryonic shield, magnified about ten times. The clear circular field (d) is the opaque area. The pellucid area (c) is lyre-shaped, like the embryonic shield itself (b). In its axis is seen the dorsal furrow or medullary furrow (a). (From Bischoff.)) The sole-shaped germinal shields of all the amniotes are still, at the stage of construction which Figure 1.134 illustrates in the rabbit and Figure 1.135 in the opossum, so like each other that we can either not distinguish them at all or only by means of quite subordinate peculiarities in the size of the various parts. Moreover, the human sandal-shaped embryo cannot at this stage be distinguished from those of other mammals, and it particularly resembles that of the rabbit. On the other hand, the outer form of these flat sandal-shaped embryos is very different from the corresponding form of the lower animals, especially the acrania (amphioxus). Nevertheless, the body is just the same in the essential features of its structure as that we find in the chordula of the latter (Figures 1.83 to 1.86), and in the embryonic forms which immediately develop from it. The striking external difference is here again due to the fact that in the palingenetic embryos of the amphioxus (Figures 1.83 and 1.84) and the amphibia (Figures 1.85 and 1.86) the gut-wall and body-wall form closed tubes from the first, whereas in the cenogenetic embryos of the amniotes they are forced to expand leaf-wise on the surface owing to the great extension of the food-yelk. (FIGURE 1.131. Embryo of the opossum, sixty hours old, one-sixth of an inch in diameter. (From Selenka) b the globular embryonic vesicle, a the round germinative area, b limit of the ventral plates, r dorsal shield, v its fore part, u the first primitive segment, ch chorda, chr its fore-end, pr primitive groove (or mouth). FIGURE 1.132. Sandal-shaped embryonic shield of a rabbit of eight days, with the fore part of the germinative area (ao opaque, ap pellucid area). (From Kolliker.) rf dorsal furrow, in the middle of the medullary plate, h, pr primitive groove (mouth), stz dorsal (stem) zone, pz ventral (parietal) zone. In the narrow middle part the first three primitive segments may be seen.)