POLYEMBRYONIC DEVELOPMENT IN TATUSIA 595 
cles, with which it is undergoing fusion. The smaller vesicle is 
similar in its general structure to the larger one, but retains a 
distinct conection with the ectodermal vesicle. 
The relation of the mesodermal vesicle to the rest of the’ chor- 
ionic vesicle is of great interest, and can be illustrated by explain- 
ing the manner in which the sections have been cut. In each of 
the specimens represented in figures 51 to 61 the sections have been 
cut so that their plane is parallel to the plane passing through the 
‘horizontal grooves’ of the uterus and perpendicular to the surface 
of the mucosa (fig. 21). We thus see that the right and left me- 
sodermal vesicles lie on those sides of the blastocyst which are 
turned towards the right and left openings of the fallopian tubes, 
respectively. 
In figure 19 the mesodermal vesicles lie to the right and left of 
the center of the blastocyst, and, as we shall see later, hold the 
same orientation as do the two primary buds of the embryos. 
Figure 59 shows a median section of a vesicle in which the two 
mesodermal vesicles have expanded until they occupy the entire 
extraembryonic cavity; but the double partition made by the ap- 
proach of their adjacent sides does not allow a communication of 
the two cavities (fig. 20). However, this condition does not exist 
throughout the entire series of sections, for in many places a por- 
tion of the partition has already broken down. Eventually it 
will entirely disappear. This final condition in the formation of 
the extraembryonic mesoderm is soon reached, and the single 
layer of flattened mesodermal cells then completely lines the space 
below the ectoderm, conforming to the various irregularities of its 
bounding walls (fig. 22). The new cavity thus formed and lined 
with the mesodermal epithelium may now be called the extraem- 
bryonic coelome. 
COMPARISON WITH OTHER FORMS 
At this point it is well to compare briefly the armadillo blasto- 
cyst with that of other forms. The early development of the 
armadillo parallels most closely that of the mammals in which the 
so-called inversion of germ layers isfound. Fernandez (’09) has 
pointed out that certain stages of the South American armadillo 
