Development of the Nine-Banded Armadillo. 379 



where it gradually passes into the surrounding entoderm. In 

 many places the mesoderm cells are beginning to migrate in between 

 the plate and the ectoderm, and especially is this true in the more 

 anterior sections (fig. 21). Tn 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, al) . 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 extend laterally to form the two wing-like processes, 

 which are to be interpreted as representing cross section of the 

 belly-stalk bands (6. 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 



