82 THE BIOLOGY OF TWINS 



Although the embryo, now represented mainly by the 

 ectodermic vesicle, at first lies at the distal pole of the 

 egg, it subsequently shifts its position until it appears 

 to be attached to the proximal pole of the egg, as 

 shown in Fernandez' figure. The explanation of this 

 apparent total reversal of position is not far to seek, for 

 there is a close analogy in this respect between Dasypus 

 and Euphractus. Referring again to Fernandez' figure 

 (Fig. 34) , we see that the Haftsiiel or primitive umbilicus 

 shows the same relation to the endodermal allantois 

 that exists in the genus Dasypus. The embryo appears 

 to have grown backward along the vesicle wall through 

 an arc of over 180 degrees. The result is that the 

 anterior end of the embryo which formerly pointed to 

 the left now points to the right; the dorsal aspect, that 

 formerly faced toward the proximal pole, now faces 

 toward the distal pole. Note the slender Nahelstrang in 

 Fig. 34, running from the junction of amnion and umbil- 

 icus to the distal pole of the vesicle, which, I believe, is 

 the equivalent of the amniotic connecting canals in 

 Dasypus. Analysis of the situation reveals the impor- 

 tant fact that each quadruplet embryo in Dasypus goes 

 through the same reversal of axis, the same backward 

 growth toward the Trager, and the same establishment 

 of a placental connection with the latter as does the 

 single embryo of Euphractus. Starting with a single 

 ectodermifc vesicle in both species, in Euphractus a single 

 apical end and embryonic axis is estabHshed, and in 

 Dasypus four or more apical ends appear. The real 

 problem of polyembryony is to account for the appear- 

 ance of four growing points in a vesicle that primitively 

 had but one. 



