Development of the Nine-Banded Armadillo. 379 where it gradually passes into the surrounding entoderm. In many places the mesoderm cells are beginning t o migrate in bet ween the plate and the ectoderm, and especially is this true in the more anterior sections (fig. 21). In this section, which shows six of the mesodermal cells, the anterior limit of the protochordal plate is represented. A very short distance in front of this the sections pass through the amniotic canal (fig. 20), which is seen to be com¬ posed of two layers, a rather thick inner ectodermal layer, and a thin outer mesodermal layer. In some places the canal is loosely connected with the underlying mesoderm of the yolk-sac, but for the most part it merely lies in contact with the latter. In sections lying posterior to the primitive pit there is nothing of especial note until we come to the region where the allantoic tube takes its origin. The mouth of the allantois is in the form of a deep groove traversing the ventral side of the anterior end of the belly-stalk (fig. 22, at). This is lined with an especially thick ento¬ derm and gradually fades out anteriorly, but posteriorly suddenly narrows down to form the tube. The mesoderm of the belly-stalk appears to ext end laterally to form the two wing-like processes, which are to be interpreted as representing cross section of the belly-stalk bands (b. b.). Externally these are covered with an epithelium, but within are composed of a loose mesodermal tissue in which run the umbilical blood vessels together with their accom¬ panying sinuses. In section the posterior amniotic process is triangular in shape, and is not much more than half the width of the belly-stalk. In sections taken through the posterior end of the embryo (fig. 23) the allantois is reduced to a slender tube, having a small lumen. The amnion is here triangular in cross section with the lower angle coming in close proximity to the allantoic entoderm. The mesoderm has much the same shape as in the preceding figure, but may be divided rather indistinctly into two portions: (1) the allantoic mesoderm which surrounds the entodermal tube, and has the cells compactly arranged; (2) the more distal wings or belly-stalk, bands through which the blood vessels run. The semidiagrammatic longitudinal section of the primitive streak stage is shown in fig. 24, and in connection with what has