POLYEMBRYONIC DEVELOPMENT IN TATUSIA 597 
taining a stage which corresponds exactly to the stage of the mouse 
shown in figure 33 of Melissinos, although specimen No. 340 (fig. 
15) may be compared with it. Figure 34 of Melissinos and my 
figure 19 can be directly compared, especially if it be kept in mind 
that the parietal layer of yolk-sac entoderm is incomplete in the 
armadillo. In the mouse the embryonic ectoderm is borne upon 
a mass of cells conecting it with the ectoplacental plate, and the 
separation of these two embryonic structures does not take place 
for some time. In the armadillo, on the contrary, the ectodermal 
mass early parts company with the Trager or ectoplacental re- 
gion, thus giving rise to an extraembryonic cavity at a very early 
stage. It is this difference in the time of appearance of the extra- 
embryonic cavity which renders difficult an exact comparison 
between the two forms throughout the subsequent history of 
development; and in support of this view we may cite the case of 
mesoderm formation. 
We have seen that the mesoderm does not make its appearance 
in the armadillo blastocyst until after the extraembryonic cavity 
has arisen, and that upon arising from that portion of the ectoder- 
mal vesicle which is turned toward the placental region, it imme- 
diately develops into an epithelial lining membrane for this cavity, 
which is thereby transformed into a true extraembryonic coelome. 
In the mouse, according to the account of Melissinos, the meso- 
derm is very early recognized as a mass of cells lying in a position 
somewhat similar to that of the mesoderm in the armadillo; that 
is, immediately ventral to the ectodermal mass, at the point of 
constriction between the embryonic ectoderm and the group of 
cells connecting it with the ectoplacental plate. Later, when the 
embryonic ectoderm and the ectoplacental plate become entirely 
separated, a cavity lined with mesoderm appears between these 
two embryonic structures. This cavity becomes then an extra- 
embryonic body cavity (the ‘mittlere Héohlung’ of Melissinos). 
When this process is completed we are presented with a condition 
quite similar to that of a relatively late but corresponding stage of 
the armadillo (cf., fig. 43 of Melissinos and my fig. 22). The es- 
sential difference lies in the fact that in the armadillo blastocyst 
there is no ectoplacental cavity; but even this difference later be- 
