596 J. T. PATTERSON 
Mulita are comparable to corresponding stages of the mouse, as 
figured by Melissinos (07); and, while a similar comparison may 
be made between the mouse and the Texas armadillo, neverthe- 
less, a closer similarity exists between the early stages of the fru- 
giverous bat Pteropus and this armadillo. Thus one of the 
youngest stages of Pteropus figured by Selenka and Géhre (’92 
pl. 41, fig. 4) is strikingly like the blastocyst shown in figure 17 of 
this paper; for in each case the embryonic ectoderm has separated 
from the trophoblast to form a spherical mass, which has become 
included within the entoderm. 
The principal feature in which they are dissimilar is seen in the 
extension of the entoderm. In Pteropus the entoderm completely 
lines the cavity of the blastocyst, forming an epithelial lining for 
the yolk-sac. In the armadillo the entoderm extends out along 
the inner side of the trophoblast for only a short distance from the 
ectoderm, and at most never covers an area of over 80° on the cir- 
cumference of the blastocyst. Consequently a closed epithelial 
sac of entoderm is not formed, and the yolk-sac cavity is bounded 
on the non-embryonic side by a single layer of trophoblastic cells, 
or chorionic ectoderm (fig. 19). 
The similarity between Pteropus and the Texas armadillo is not 
confined to this early period, but is also seen in later stages; es- 
pecially is this true with reference to the formation of the amniotic 
cavity. In each, the solid sphere of ectoderm becomes hollowed 
out through the disintegration or vacuolization of the core to form 
the primary amniotic cavity (cf. fig. 18 with fig. 6 of Selenka and 
Gohre). Finally, in the condition of the mesoderm the two forms 
show several points of similarity. 
In the blastocysts of the mouse and of the armadillo are also 
to be seen many points of similarity, though the resemblance is 
here less striking than in the preceding case. The figures of 
Melissinos (’07) are very similar to several of the stages shown in 
this paper. His figure 31 shows a stage directly comparable to 
our specimen No. 311 (fig. 11); and his figures 33 and 34 illustrate 
the manner in which the embryonic ectoderm is pushed out into 
the general cavity of the blastocyst, carrying before it the visceral 
layer of entoderm. In the armadillo I have not succeeded in ob- 
