XI, B, 3 Ruth: Development of Twins 113 



Each embryo is in the eight-somite stage of development. The 

 medial somites lie immediately anterior to the medullary groove. 

 The somites are seen much more plainly from the ventral than 

 from the dorsal side (Plate II) . 



SET III 



Dorsal side. — These tv^^ins are in the twelve-somite stage of 

 development and appear to be perfectly normal. The heads 

 and caudal portion of the embryos are separated from each 

 other, while in the thoracic region they are joined. This type 

 of twins is known as thoracopagus. This set of twins I recon- 

 structed, and in another paper I shall describe them in detail. 

 The heads are parallel with each other, while the caudal ends are 

 separated, forming an angle of about 45 degrees. Ventral to 

 the head processes is the proamnion, which is thin and transpar- 

 ent. The area pellucida regularly surrounds the embryos on all 

 sides. The cephalic portions of the embryos are uniformly 

 developed. On the tip of each head is a depression — ^the an- 

 terior neuropore. The medullary grooves are closed except for 

 two small portions — one in the region of the somites, and the 

 other along the caudal portion of each embryo. The latter 

 portion is quite wide and expanded at the distal end. The fore- 

 and hind-brain are separated from each other by a slight con- 

 striction, the former being considerably more expanded in a 

 lateral direction. The hind-brain has about the same length as 

 the combined length of the fore- and mid-brain. Between the 

 two embryos in the cardiac region is a saccular protuberance into 

 which blood vessels empty. The omphalomesaraic veins and a 

 large median vein are the principal veins that empty into the 

 primative heart. 



Ventral side. — The head processes can be distinctly seen 

 through the proamnion. Posterior to the heads the entoderm 

 is thro\\Ti into a fold which forms a right angle with the axis 

 of the embryo. These folds are caused by the omphalomesaraic 

 veins as they approach the embryos and terminate in the cardiac 

 region. Immediately posterior to the omphalomesaraic veins 

 is the posterior opening of the fore-gut or the fovea cardiaca. 

 The two openings are separated from each other by a ventral 

 bulging which is formed by the median vein as it approaches the 

 embryo from a caudal direction (Plate I, figs. 1 and 2). 



SET IV 



The bodies of these two embryos lie in the same linear axis. 

 One component is in a fairly good state of preservation, while 



