mi, b, 4 Ruth: Reconstruction of Duck Twins 203 



its appearance (Plate II). This, like the other, is also lost in 

 the interembryonic tissue at a distance slightly more caudad. 



DUCK TWINS SET III 



Reconstruction of duck twins set III shows the foreguts and 

 vascular system symmetrically developed in every detail (Plate 

 II) . In the region of the thorax, abdomen, and caudal extremity 

 the ectoderm and entoderm are continued from one embryo to 

 the other. The dorsal aortse are all distinctly and uniformly 

 developed, though their continuation into the vitalline arteries 

 cannot be traced. In the region of the fovea cardiaca the venous 

 blood vessels form a median confluence by the union of the two 

 omphalomesaraic veins with the median vein, which approaches 

 the embryos from a caudal interembryonic direction. The large 

 median vein is formed from a series of smaller veins beginning 

 symmetrically from eight smaller veins. Extending cephalad to 

 the venous confluence are two tubular projections. The tubular 

 hearts are symmetrically developed. Both hearts, as they ap- 

 proach the cephalic portion of the foreguts, divide into two 

 ventral aortse, which loop around the lateral sides of the pharynx 

 to form the dorsal aortse. The dorsal aortae terminate rather 

 abruptly in the region of the venous confluence on the posterior 

 side of the foregut. No vitelline arteries appear to be present. 

 The medullary tubes and grooves are entirely separate and dis- 

 tinct from each other throughout the entire extent of the em- 

 bryos; both are symmetrically developed. On the ventral side 

 of each medullary tube is a notochord. The foreguts are uni- 

 form in every detail. Each component in this set of twins is 

 separately developed, with the exception of the venous confluence. 

 Here the two embryos appear to have a vascular chamber in 

 common (Plate II). All other structures at this stage of de- 

 velopment are duplicated. 



DISCUSSION 



The two sets of twins have a number of developmental parts 

 in common. However, in set V the medial dorsal arteries are 

 only partially developed, while the lateral have already joined 

 with the ventral aortae. In the older twins the blood vessels 

 have apparently assumed a fixed position in relation to the 

 embryo, while in the younger twins they are in a migrating 

 stage, that is, in the younger twins the heart anlage has not yet 

 become tubular in several areas. In both twins there is present 

 a venous confluence at the proximal end of the tubular hearts 

 formed by two omphalomesaraic veins and a median vein. 



