Development of the Nine-Banded Armadillo. 393 



The conjecture that the compound placenta of T. novemcinctum 

 has been derived without any fusion of four embryonic vesicles 

 from a condition similar to that described bj^ Chapman for Dasy- 

 pus sexcinctus, is very tempting in view of the evident close rela- 

 tionship of the two species and the striking resemblance that exists 

 between them in the details of the placenta, umbilicus and other 

 structures. This if true would furnish one of the most cogent 

 proofs of polyembryony, since we find in the more highly special- 

 ized species a quadruple placenta, which at a rather early period 

 closely resembles the definitive placenta of a more primitive 

 species that gives birth to single young or to twins. ^ 



VI. History of the Amnion 



From Fernandez's description of his earliest stage it is clear that 

 the common amniotic cavity is at first the hollow of the ectoder- 

 mic vesicle, which, through the inversion of germ layers, has come 

 to lie within an envelope of entoderm. Regional differentiation 

 .of this ectodermic vesicle produces the ectodermal portions of the 

 embryonic primordia, which are at first contained within a single 

 vesicular amniotic cavity. Subsequently the individual embryos 

 sink into pockets in the floor of the common amnion, which has 

 evidently become fused to the walls of the yolk-sac at the cervix 

 pole of the embryonic vesicle. The posterior end of each embryo 

 has become fixed by means of the primordium of the belly-stalk 

 to the margin of the Trager, and consequently, as the yolk sac 

 gradually increases in size, the embryos are drawn away from the 

 common amnion, retaining connection with it only by means of 

 slender tubes, the amniotic connecting canals (figs. 12 and 14). 

 It has been shown that each pair of embryos withdraws from the 



^ We are informed by Mr. Robert D. Carson, superintendent of the Philadelphia 

 Zoological Garden, that a female six-banded armadillo in captivity gave birth to: 



1. A single male, on May 10, 1901. 



2. Twin males, on April 6, 1902. 



3. Twins (male and female), on July 19, 1902. 



