Development of the Nine-Banded Armadillo. 381 



In the five somite embryo (fig. 30) the neural folds have not yet 

 coalesced to form the brain vesicle, and consequently the neural 

 groove is open throughout its entire length. The posterior ends 

 of the neural folds embrace the much reduced primitive streak. 

 The embryos are bounded laterally by an area pellucida, which is 

 rapidly being invaded by the blood cords. 



In sharp contrast to this embryo is the individual from the other 

 pair showing seven somites (fig. 31), and unless one were from the 

 first aware that they were members of the same set of embryos, 

 one would not so classify them. There are really only six and one- 

 half somites in this embryo, for the most anterior or cephalic 

 pair is connected witn the head mesoderm and is somewhat smal- 

 ler than the succeeding pairs (fig. 15). There is a slight indica- 

 tion of an eighth pair being cut off from the anterior end of the 

 unsegmented paraxial mesoblast. 



The amnion has undergone several marked changes, chief among 

 which are (1) its enlargement in the cephalic region of the embryo 

 and (2) its reduction in width at the level of the distal part of the 

 belly-stalk. In this stage the neural folds have risen up and coal- 

 esced to form a portion of the neural tube. The point where the 

 fusion first occurs is at the level of the mid-brain region, and from 

 this place it progresses both backwards and forwards. The anter- 

 ior progress of the union, however, takes place rather slowly and 

 the final closing on the under side of the fore-brain to form the 

 neuropore does not occur until a period much later than this. 



At the posterior end of the diverging folds the reduced primi- 

 tive streak is seen as a broad plate, which in the mid-ventral 

 region is slightly concave, and by transmitted light appears to be 

 decidedly thicker than the lateral portions. The notochord is 

 seen to arise from the anterior end of the primitive streak and 

 to extend forward between the folds. At the point of origin 

 of the notochord the primitive streak is unusually thick, forming 

 a distinct primitive knot, just back of which is the suggestion of 

 a primitive pit. At the posterior end of the primitive streak the 

 entodermal allantois is faintly visible. It extends backward 

 lying beneath the floor of the posterior amniotic process, and falls 

 far short of reaching the tip of the latter. 



