614 J. T. PATTERSON > 
these two embryos are shown, and they lie each at the end of 
a common bay by means of which their amniotic cavities are 
placed in communication with the cavity of the common amni- 
otic vesicle. This condition is of the greatest significance, since 
it indicates the common origin of the pair of embryonic tubes 
from the left-hand primary bud. 
Figure 83 shows all four of the embryonic rudiments in sec- 
tion lying on the sides of the wall of the blastocyst, and facing 
on the inner side of the large extraembryonic cavity. 
Figure 84 is taken about half way between the posterior ends 
of the embryonic tubes and the base of the chorionic vesicle. 
Here the wall of the blastocyst is composed of the typical 
structures, entoderm on the outside and mesoderm within. The 
latter is rapidly becoming vasculated, especially in the regions 
lying directly posterior to the embryonic tubes. 
Soon after the stage just referred to, the chorionic vesicle 
begins a very rapid expansion, and, in doing so, first assumes a 
bulb-like shape (fig. 5). The vesicle is united to the mucosa by 
an annular zone of thickened trophoblastic ectoderm, the so- 
called Triger (fig. 23), and from the edge of this the yolk sac 
entoderm extends upwards to form the outer layer of the chori- 
onic wall, the chorionic ectoderm having long since disappeared. 
The inner layer of the chorion everywhere consists of mesoderm. 
The embryonic portion of the vesicle consists of the relatively 
small, common amniotic vesicle (fig. 5, C A. V.), from the right 
and left sides of which spring a pair of small connecting canals. 
Each canal places the cavity of the amniotic vesicle in direct 
communication with the amniotic cavity of the embryo. It 
will be recognized that these canals are the elongated proximal 
parts of the original ‘secondary buds’, and that the embryonic 
rudiments are the distal portions of such buds. This is the 
reason why the canals spring in pairs from the vesicle. How- 
ever, there is considerable variation in the relation of the pair 
of canals to the vesicle. The one presented in figure 5 is rather 
unusual, in that the amniotic vesicle is greatly elongated, instead 
of spherical, and the two canals of a pair arise very close to- 
gether from .the end of the vesicle. The usual condition shows 
