CAUSES OF TWINNING IN ARMADILLOS 117 



individual embryos may pass in order to reach the 

 opposite end of the vesicle, where lies the Trager. Each 

 embryo in our twinning species, in thus migrating to the 

 placental pole of the vesicle, behaves just as if it were 

 the only embryo in the vesicle. It seems clear then 

 that, unless we are prepared to call the outpouching of 

 the non-twinned embryo of Euphractus a ''bud," the 

 term "bud" for the homologous structure of Dasypus 

 is entirely a misnomer. It seems strange also to think 

 of embryonic primordia budding at the tail end, as they 

 would be doing if this is budding; for in typical cases 

 of budding it is implied that the budding region is a new 

 apical region or head region. It is quite certain that in 

 all of Stockard's work he considers his ''buds" as new 

 head regions. 



When the embryos first begin their backward migra- 

 tion toward the Trager, the paired embryos of the right 

 side remain broadly attached by a band of ectoderm. 

 The same is true of the pair on the left side. They 

 therefore migrate together for a short distance and are 

 in the same amniotic canal. Soon, however, the connect- 

 ing band thins out and breaks apart in forklike fashion, 

 and the two embryos proceed to grow and migrate down 

 two distinct canals, tail first, as though switched back- 

 ward onto diverging tracks. There is left, therefore, 

 for a little distance from the common amniotic vesicle, a 

 common canal which soon splits into the two individual 

 connecting canals. One can always identify the twin 

 products of one side of the vesicle by the fact that their 

 individual connecting canals thus unite before entering 

 the common amnion. It is this forking apart of the 

 posterior ends of the embryos that gives the appearance 



