120 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



the right of the midhne and its extremity is saccular. This saccular portion 

 becomes the gall bladder. Several ducts connect the diverticulum with the liver 

 cords. One of these persists as the hepatic duct which joins the cystic duct of the 

 gall bladder. The portion of the diverticulum proximal to this union becomes the 

 common bile duct, or ductus choledochus. The ventral pancreas arises from the 

 common bile duct near its point of origin (Fig. 123). It is directed dorsad and 

 caudad to the right of the duodenum. The dorsal pancreas arises more caudally 

 from the dorsal wall of the duodenum and its larger, lobulated body grows dor- 

 sally and cranially (Figs. 123, 127 and 140). Between the pancreatic anlages 

 courses the portal vein. In the pig, the duct of the dorsal pancreas persists as 

 the functional duct. 



Intestine. — Caudal to the duodenum, the intestinal loop extends well into 

 the umbihcal cord (Figs. 122 and 123). At the bend of the intestinal loop is the 

 slender yolk stalk. The cephalic Hmb of the intestine hes to the right, owing to 

 the rotation of the loop. The small intestine extends as far as a shght enlarge- 

 ment of the caudal hmb of the loop, the anlage of the ccecum, or bhnd gut. This 

 anlage marks the beginning of the large intestine (colon and rectum). The 

 intestinal loop is supported by the mesentery which is cut away in Fig. 122. The 

 cloaca is now nearly separated into the rectum and urogenital sinUs. The ca\dty 

 of the rectum is almost occluded by epithelial cells (Lewis) . 



Urogenital System. — The mesonephros is much larger and more highly dif- 

 ferentiated than in the 6 mm. embryo (Figs. 120 and 124). Along the middle 

 of its ventro-median surface the genital fold is now more prominent (Fig. 122). 

 In a ventral dissection (Fig. 124) the course of the mesonephric ducts may be 

 traced. They open into the urogenital sinus, which also receives the allantoic 

 stalk (Fig. 122). 



The metanephros, or permanent kidney anlage, Hes just mesial to the um- 

 bilical arteries where they leave the aorta (Fig. 123). Its epithelial portion, 

 derived from the mesonephric duct, is differentiated into a proximal, slender duct, 

 the ureter, and into a distal, dilated pelvis. From this grow out later the calyces 

 and collecting tubules of the kidney. Surrounding the pelvis is a layer of con- 

 densed mesenchyma, or nephrogenic tissue, which is the anlage of the remainder 

 of the kidney. 



Blood Vascular System.— T^e Heart. — In Fig. 125 the cardiac chambers of 

 the right side are opened. The septum primum between the atria is perforated 

 dorsad and cephalad by th.Q foramen ovale. The inferior vena cava is seen opening 

 into the sinus venosus, which in turn communicates with the right atrium through 



