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 Gohre ('92 

 pi. 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 severarpoints 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 hown 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- 



