No. 561] 



THE NINE-BANDED ARMADILLO 



527 



thickenings of the ectodermal vesicle. These two pri- 

 mary embryonic buds arise in connection with the dual 

 centers of origin of the mesoderm, each appearing 

 directly beneath a primary mesodermal pouch. These 

 two primary buds elongate and soon divide at the tip into 

 paired outgrowths, which constitute the primordia of the 

 two pairs of embryos. The embryos develop on the 

 inside of the inner vesicle and are consequently in a com- 

 mon ectoderm-lined, fluid-filled cavity, which is a sort of 

 common amnion. Subsequently the separate embryos sink 

 into the floor of the common amnion and retain their con- 

 nection with the latter only by slender amniotic connect- 

 ing canals, which gradually shrivel up and disappear. 

 An early somite stage with the common amnion and the 

 connecting canals still intact is shown in Fig. 5, which 

 also illustrates the attachment of the four foetuses to the 

 Trager by means of the allantois and the belly-stalk 

 bands which constitute the primitive umbilicus. The 

 saucer-shaped Trager or primitive placenta develops 

 from the part of the trophoblast which originally formed 

 the point of attachment for the vesicle. This area has not 

 been invaded by the entodermal vesicle, but is rein- 

 forced directly by mesoderm, which invades the maternal 

 mucosa and produces primitive villi, that are at first in 

 the form of blunt ridges, but later take on the form of 

 flat scales (see Fig. 5), and subsequently assume the 

 typical arborescent form of definitive placental villi. The 

 subsequent development of the embryos is of little inter- 

 est except to the specialist and need not be referred to 

 here. The history of the placenta, however, is of un- 

 usual interest in that it illustrates the futility of attempt- 

 ing to use the special types of placentation as criteria of 

 animal affinities. The early placenta as shown in Fig. 5 

 is a single discoid structure. Subsequently the points of 

 attachment of the four umbilical cords become areas of 

 rapid placental development and the parts of the Trager 

 in between them almost lose their villi. At this stage the 

 placenta consists of a set of four separate discs. As 



