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 'Verbindungsrohre' 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. 



