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 Trager 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 Trager epithe- 

 lium (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 awa;y. 



