Development of the Nine-Banded Armadillo. 387 



one-third of the area of the embryonic vesicle (the remainder 

 consisting of the yolk-sac region), that the embryos are attached 

 to the Trager by paired bands of mesoderm, equivalent to the 

 belly-stalk of the primates, and that the central area of the Trager 

 is freer from thickenings than the periphery. 



The function of the Trager or primary placenta appears to be 

 not so much nutritive as merely adhesive, since tfiere are at this 

 time no blood-vessels in it by means of which nutriment might 

 be conducted to the embryos. It is highly probable that whatever 

 nutriment reaches the embryos comes to them by a process of 

 osmosis through the thin wall of the yolk-sac region of the 

 vesicle. 



The formation of the secondary placenta occurs entirely within 

 the confines of the Trager and involves at tne beginning practi- 

 cally its whole area. A very instructive stage in the development 

 of the placenta is seen in vesicle 18, (figs. 14 and 15). Here the 

 Trager epithelium has been pushed out into short scaly villi, 

 which show a tendency to overlap one another as well as the mar- 

 gin of the yolk sac region. These protuberances have been in- 

 vaded by a stroma-like mesencbyme, which has arisen from the 

 original thin mesodermal epithelium lining both Trager and yolk- 

 sac regions of the vesicle. The free ends of the scale-like villi 

 are tipped with masses of solid gland-like tissue derived by the 

 breaking up of the branching cords of earlier stages into numerous 

 knots which are carried out to the extremities of the individual 

 villi. Although the general Trager epithelium which surrounds 

 the villi has persisted in the form of a rather thick syncytial layer 

 the knots are bare of covering except for the presence of an ex- 

 tremely thin layer of much flattened and scattered cells. The 

 knot cells therefore are in a position to come into most intimate 

 contact with the uterine tissues and probably serve as organs 

 of penetration, softening the maternal tissues by means of a 

 secretion and forcing open a path for the villi, in much the same 

 way as the diamond tips of drills cut away the harder materials 

 and open up a path for the shaft. These Trager knots forming the 

 tips of the villi appear to persist throughout almost the entire 

 foetal life in a form practically identical with that just described. 



