624 J. T. PATTERSON 
mals, but in the blastocyst of the armadillo its loss results in 
bringing the yolk-sac entoderm to the outer surface. From now 
on the increase in size of the blastocyst is evidently brought about 
through the growth of this entodermal portion of the wall of 
the ovum, as can be determined by comparing successive 
stages. But after the vesicle has attained a diameter of from 
4 to 5 mm. (fig. 5) the entodermal portion does not expand to 
any great extent, and the chorionic vesicle owes its further growth 
to the extension of the Trager or placental portion of the cho- 
rionic wall. 
As the Trager area increases in extent the yolk-sac is carried 
farther and farther toward the cervix end of the uterus, and in the 
advanced stages of gestation it forms a cap at the tip of the cer- 
vix end of the chorionic vesicle. However, it becomes partly 
covered by an overgrowth of Trager tissue, which arises at the 
boundary line between the yolk-sac and the Trager, and extends 
posteriorly as a free margin which later becomes fused with the 
wall of the cervix. In vesicles which contain 30 to 35 mm. 
embryos, the yolk-sac portion of the chorion protrudes through 
the thickened ring-like, placental overgrowth as a clear trans- 
parent membrane. 
If a chorionic vesicle from which the chorionic ectoderm has 
already disappeared be examined, it is found that the entoderm 
is attached to the upper edge of a mass of Traiger tissue which 
lies Just inside the basal fragment of trophoblast (figs. 70-74). 
In tracing back the origin of this mass one finds that it arises as 
a thickening on the inner surface of the Trager zone, just below 
the margin of the entoderm. It can be recognized in very early 
stages as a small mass of actively dividing cells which lie in the 
angle formed by the Trager zone and the potential Traiger epithe- 
lum (fig. 55, on left). The thickening gradually increases in 
volume (figs. 33, 34, m), and apparently involves the entire inner 
surface of the Trager zone, and thus comes to form at its crest a 
fusion with the margin of the entoderm (fig. 23, on right). At 
the same time the outer layer of cells of the Trager zone becomes 
split off from the mass (fig. 23, on left), and thus forms the basal 
fragment when the chorionic ectoderm breaks away. 
