592 J. T. PATTERSON 
ed cells which are, for the most: part, of entodermal origin, but 
which will soon disappear. 
The entodermic vesicle is completely developed. Its upper or 
embryonic side is two or three cells thick, while its lower side is 
but a single cell deep. On this side is a small opening or pore 
which places the amniotic cavity in communication with the 
extraembryonic cavity. This pore is found in but four sections, 
and no other vesicle shows it, thus leading one to suspect that its 
existence is more or less accidental, due to the manner in which 
the vacuolization took place. In blastocyst No. 332 the vacuo- 
lization occurs towards that side of the ectodermal mass which is 
nearest to the mucosa, and in the present specimen we may sup- 
pose that the excentric position of the vacuolization has resulted 
in perforating the lower side of the vesicle. 
There is, of course, another very plausible explanation, namely, 
that the opening appears in all of the vesicles, but soon closes 
thus accounting for the fact that it is never seen in the older 
stages. This view receives support from the work of Fernandez 
(09) on Mulita. The youngest stage secured by Fernandez is 
about in the same state of development as. this specimen of the 
Texas armadillo, and presents the same structural relations. Fer- 
nandez states that the cavity of the ectodermal vesicle and what 
he calls the ‘Trager cavity’ are connected by a small pore which 
he compares to the ‘Verbindungsréhre’ in the mouse (Melissinos 
07). His ‘Trager cavity’ represents the same space that I have 
called the extraembryonic cavity in T. novemcincta. I have so 
named this cavity because later, when it becomes lined with meso- 
derm, it is recognized as a true exocoelome. It should be pointed 
out here that what Fernandez terms the Trager cavity in his sec- 
ond youngest state (Fernandez ’09, text figure 2) I have regarded 
as an artifact (for reasons to be presented later) and consequently 
it can not be compared with the similarly named cavity of his 
youngest stage. 
In this mode of producing a vesicle of the ectodermal sphere is 
to be recognized an amnion formation through a process of vac- 
uolization; for the entire subsequent history of development 
demonstrates that the cavity thus formed is an amniotic cavity. 
