Development of the Nine-Banded Armadillo. 375 



Hum. At the extreme lower end there is a small cap-like area 

 where the primitive attachment knots or cords of the Trager epi- 

 thelium are beginning to disappear. The other region occupies 

 the upper two-thirds of the vesicle and differs from the preceding 

 both in its greater transparency and in the complete absence of a 

 trophoblast. This region is the yolk-sac of the inverted type, 

 and consequently is covered with the entoderm. It is rather in- 

 distinctly divided into two portions : (1) the central zone occupied 

 by the embryos and their vascular areas, and (2) the cap-like 

 upper third in which the almost complete transparency is ob- 

 structed by the presence of the common amnion and its connecting 

 canals. 



Two of the embryos lie on the upper side (corresponding to the 

 ventral side of the uterus) and two on the lower side of the vesicle. 

 Each embryo is connected with the Trager region by a rather 

 broad band, the belly-stalk, and is surrounded by an amnion. 

 Since there is an inversion of germ layers, the embryos when 

 viewed from the outside of the vesicle are seen from their ventral 

 aspects; hence, the posterior portion of each amnion is invisible 

 except as seen through the semi-transparent embryo. Anteriorly, 

 however, the lateral margins of the amnia are clearly distinguish- 

 able and are seen to pass forward as the tube-like, amniotic, con- 

 necting canals. These lie on the inner or mesodermal surface of 

 the yolk-sac, to which they are loosely attached, and in passing 

 forward they converge and finally enter the common amnion. 

 They do not communicate with this by four distinct openings, 

 but by two, for just before reaching it, the canals belonging to the 

 dorsal and left lateral embryos unite to form a single tube, as do 

 also those belonging to the ventral and right lateral embryos. 

 As will be pointed out in another section, this fusion of the canals 

 is an indicatiqn of the pairing of the embryos since the union in 

 each case is between individuals of a pair. 



The common amnion at this stage is a comparatively small 

 vesicle lying at the extreme cervix end of the vesicle. The man- 

 ner in which this condition has been evolved from that seen in 

 the second stage of Fernandez is not difficult to figure out. On 

 the one hand, the cavity of the embryonic vesicle has undergone 



