Development of the Nine-Banded Armadillo. 377 



Lateral to the embryo is seen the beginning of the yolk-sac or 

 vitelline circulation. At this time the blood islands are well 

 developed and incipient blood vessels are represented by a net- 

 work of anastomosing cords of mesoderm. About midway be- 

 tween any two contiguous embryos there is a band-like area extend- 

 ing from the Triiger to the upper limit of the area vasculosa. 

 The band represents the region where the boundaries of the vas- 

 -cular areas of adjacent embryos come together, and thus corres- 

 ponds to the sinus terminalis of other forms, except that it is 

 double in composition. At the anterior margin of the vascular 

 area of each embryo the sinus terminalis tends to form the arc 

 of a circle, a tendenc}^ which, if not inhibited by the crowding of 

 four embryos, would result in the production of a circular sinus 

 exactly as in other forms. As a result of this retardation by crowd- 

 ing the anterior margin of the vascular zone of the four embryos 

 is in the form of a series of scallops. 



For an appreciation of the condition of the germ layers it is 

 necessary to turn to a study of representative sections. In the 

 most typical of these, such as that taken through the primitive 

 pit, the neural portion of the ectoderm is thick and has the general 

 appearance of that of corresponding stages of other forms (fig. 

 19). The outer ends of the section curve decidedly upward, 

 especially the one on the right, but for the most part this is due 

 to the fact that the embryo conforms to the general curvature of 

 vesicle. At the ends of the section the medullary plate turns up- 

 ward to form the amniotic ectoderm, which is composed o^ a single 

 layer of cells. 



In the central part of the section the entoderm is composed 

 of rather flattened cells, which, however, remain distinct from 

 the overlying mesoderm. Beyond the limits of the primitive 

 streak it becomes thicker and its cells are cuboidal in shape. 

 It must be kept in mind that the entoderm actually forms the 

 outer surface of this region of the vesicle; for the trophoblast has 

 practically disappeared and there are found only a few of its cells 

 scattered here and there along the outer surface of the entodermal 

 layer. 



