Development of the Nine-Banded Armadillo. 383 



geal pouches ( ph. p.) are seen as bays of entoderm lying on each 

 side of the neural tube. 



The mesoderm in this region is in two rather distinct forms; 

 the outer portion is epithelial in character and conforms to the 

 general contour of the entire surface of the section; and the other 

 part is composed of mesenchyme and lies to each side of the im- 

 perfectly formed brain vesicle, and consists of scattering stellate 



cells. 



The medullary plate gradually grows narrower as one passes 

 backward until the region of the somites is reached, where its 

 width is about one- third that of the entfre embryo. The margins 

 of the entoderm have almost grown together beneath the noto- 

 chord. The mesoblastic somites are partlj^ constricted off from 

 the lateral plates, which are undergoing the process of splitting 

 into the somatic and splanchnic layers, between which is the weak- 

 ly developed coelome. 



In the region of the proximal part of the allantois (fig. 28) the 

 belly-stalk bands are very much folded, having their outer margins 

 turned up to form the scrolls that were noted in fig. 15. The 

 umbilical blood vessels in the bands are well organized and are 

 lined with an endothelium. The only other structure worthy 

 of special mention is the posterior amniotic process which is re- 

 duced to a small flat tube. 



The final section of this series to be considered here is one taken 

 through the posterior end of the amnion (fig. 29). The amnion 

 and median posterior portions of the belly-stalk bands are con- 

 nected by a rather slender stalk with the Trager (ms. co.). The 

 exact nature of the Trager will be considered in another section, 

 and it remains here merely to point out that the original primitive 

 knots are being rapidly transformed into villi. 



The longitudinal section of the seven somite embryo (fig. 25) 

 should be compared with that of the primitive streak stage, in 

 order to bring out the most significant changes occuring in devel- 

 opment. The notochord lies exposed throughout the greater 

 part of its length, but at each end it is covered beneath with the 

 entoderm. At the posterior end, where the notochord is covered 

 over, the entoderm is seen to turn back on itself for a short dis- 



