POLYEMBRYONIC DEVELOPMENT IN TATUSIA 623 
reason I have preferred to call it by the name of sub-Triger 
cavity. 
A further examination of the figures on plate 7 will show that 
at this stage of development the blastocyst is held to the mucosa 
by the Trager zone alone, so that in order to free it from the 
uterine wall it is merely necessary to cut this band of tissue at a 
short distance beyond the edge of the vesicle. 
The transition from the Trager stage or primary placenta to 
that of the secondary placenta, or a condition in which distinct 
villi are present, is very gradual. The first villi arise from that 
portion of the chorionic wall which corresponds to the Trager 
zone; that is at the attached margin of the chorionic vesicle. 
From here they successively appear further and further toward 
the central area of the Trager epithelium, until its entire surface 
becomes studded with them. However, the villi in the central 
region do not become so long or so highly developed as those that 
are situated towards its peripheral parts. 
Each villus starts as a thickening in the Trager epithelium, 
and soon becomes a mass of cells protruding from the epithelial 
surface. At first it is flat or disc-shaped and seems to serve as an 
adhesive pad, but later it elongates into a true Trager cord, 
which may become very much branched. These cords later 
become invaded by a stroma-like mesenchyme, developed from 
the mesodermal epithelium which directly overlies the Trager 
epithelium.’ 
In order to understand the changes which take place in the 
upper free portion of the chorionic wall it is necessary to recall 
that all of the trophoblast lying above the margin of the ento- 
derm sooner or later sloughs off, leaving only a fragment-like 
base, which in section can often be seen protruding from the 
side of the vesicle at a short distance above the uterine surface. 
This fragment is clearly shown in all of the figures of plate 9. 
The portion of the trophoblast which thus breaks away cor- 
responds to the chorionic ectoderm in the vesicle of other mam- 
8 For a more detailed account, and for figures of the structure of the villi, see 
Newman and Patterson, 710. 
