202 The Philippine Journal of Science 1918 



embryos and pointing caudad. The former is apparently the 

 position of the omphalomesaraic, and the latter is apparently 

 the position of the median vein. The posterior openings of the 

 foregut are separated from each other by a rounded, narrow 

 partition. On cross section a tubular vessel occupies this space 

 in the partition, separating the posterior ends of the foregut. 

 At the extreme tip of the caudal extremity, remains of the prim- 

 itive streak are still present. 



Dorsal side. — The heads project slightly dorsad to the pro- 

 amnion. They are distinctly separated from each other for a 

 short distance where the ectoderm becomes continuous from 

 one herd to the other. On the median side of the heads there is 

 a small cleft where the medullary groove has failed to close 

 entirely. This marks the position of the anterior neuropore. 

 The medullary groove has closed over in the cephalic region, 

 caudad to the anterior neuropore in both embryos; however, it 

 again becomes open and remains so throughout its entire extent. 

 The caudal portion of the medullary groove in the right embryo 

 is slightly more expanded than in the left. The somites are 

 regularly developed. The medial ones are somewhat larger than 

 those on the lateral sides of the embryos. 



Reconstruction of duck twins set V. — This set of twins is 

 somewhat younger than set III. The foreguts are symmetrically 

 developed, the openings of which are at first common to both com- 

 ponents, but they are soon divided and separated from each other 

 by a fold of mesoderm. The tubular hearts are indistinctly de- 

 veloped and appear to be double at the middle and caudal third. 

 The omphalomesaraic veins join the tubular hearts, the left vein 

 having a position much more cephalad than the one on the right 

 side. Vaso formative cells link the one tubular heart with the 

 other. On the dorsal side several median veins can be seen lying 

 on the medial anterior wall between the two pharynges. The 

 venous confluence can be made out rather indefinitely ; it is not 

 so distinct as in set III. As the single tubes approach the 

 cephalic end of the pharynx, the one on the left component turns 

 abruptly laterally and downward to gain the dorsal side of the 

 pharynx, where it becomes the dorsal aorta. The tubular heart 

 in the right component passes obliquely and laterally over the 

 most cephalic portion of the pharynx to gain the posterior side. 

 Both dorsal aortse pass caudad along the lateral and dorsal side 

 of the pharynx and are lost in the area pellucida after leaving 

 the embryos. On the dorsal side of the pharynx some distance 

 above the opening of the foregut a median dorsal vein also makes 



