Development of the Nine-Banded Armadillo. 397 



and continue to the body of the embryo as a single somewhat 

 flattened cord. The forked connection between the cord and the 

 vesicle is maintained as a characteristic feature of the placenta- 

 tion. In the definitive condition the umbilicus measures from 18 

 to 20 cm. in length and about 1 cm. in greatest diameter. The 

 veins are longer than the arteries and take an open spiral course 

 along the flattened edges of the cord. 



VIII. PAIRING OF THE EMBRYOS 



In our preliminary paper attention was called, in treating of the 

 nearly complete identity of the four embryos, to indications of 

 a still closer resemblance between the individuals of the right 

 and left hand pairs. In attempting to derive the four embryos 

 from the blastomeres of the four-cell stage the following suggestion 

 was offered: "This possible interpretation receives a striking 

 confirmation in the fact that the four embryos can be arranged 

 into two pairs, the individuals of which approach almost complete 

 identity; and these identicals are not only adjacent to each other 

 but are also attached to placental discs that are closely united. 

 If all four embryos are derived from a single egg, this is exactly 

 what we should expect to find; for surely the individuals derived 

 from one of the blastomeres of the two-cell stage ought to be 

 more nearly similar to each other than to the individuals of the 

 other blastomere." 



The subsequent acquisition of a large amount of additional 

 data has served only to strengthen our conviction concerning this 

 strong tendency toward pairing among the four embryos: a 

 tendency that expresses itself in the method of separation of the 

 embryos from the common amnion ; in the fusion of the four discoid 

 place'ntal areas into two double lateral discs ; in the different rates 

 of development seen in the embryos of a single vesicle; and in the 

 closer resemblance, as a rule, between the paired embryos of 

 one double placental disc than between the embryos in general. 



The forked arrangement of the amniotic canals, as was pointed 

 out in connection with vesicles 10 and 18, shows that the embryos 

 retreat from the common amnion in pairs and that only when at 



