142 EDWARD MCCRADY, JR. 



like the ventral half of the arch, and the impression is thus 

 produced that the pulmonary artery is a branch from the 

 middle of the sixth arch. Actually the sixth is entirely dorsal 

 to the origin of the pulmonary, as pointed out in connection 

 with stage 30. 



The first seven intersegmental arteries become plexiform 

 and develop longitudinal anastomoses with each other dorsal 

 to the aorta. The first six lose their connections with the aorta, 

 and the anastomoses plus the remnants of the intersegmentals 

 form the vertebral arteries in stage 31. The seventh inter- 

 segmental artery, from which the vertebral now appears to 

 arise, is at the level of the limb bud and becomes enlarged to 

 form the subclavian artery. 



Changes in the pancreas. In stage 31 the dorsal pancreatic 

 diverticulum loses its connection with the duodenum, so that 

 the anlage of the duct of Santorini is lost, and correspondingly 

 no accessory duct appears in the adult. The ventral pancre- 

 atic diverticulum has migrated away from the angle where it 

 originally formed, and now appears as a branch of the hepatic 

 .diverticulum or ductus choledochus. It is not yet in con- 

 tact with the dorsal pancreas, which is thus completely de- 

 tached from the digestive tract at this time. With the ex- 

 ception of the absence of the duct of Santorini this stage is 

 like that of a 10-mm. human embryo. In figure 45, J, the 

 ventral pancreatic bud is not well shown, as it is almost com- 

 pletely hidden by the duodenum. 



Pleuroperitoneal membranes. The anterior end of the 

 pleuriperitoneal cavity is roughly pyramidal in shape. In 

 cross section it appears triangular with the mesonephros at 

 the apex of the triangle, the lung at the medioventral corner, 

 and a new fold of mesodermal tissue at the lateroventral 

 corner (fig. 45, C). If traced anteriorly this fold, the pleuro- 

 peritoneal membrane, can be followed to the anterior apex of 

 the pyramid where it merges with the mesonephric fold. In 

 the middle sections the inner edge of the fold is directed 

 toward the interval between the lungs and the mesonephros; 

 and at its extreme caudal end it is continuous with the 



