122 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



diverticulum. It lies to the right of the midplane 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. 1 23). 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, lobu- 

 lated body grows dorsally and cranially (Figs. 123, 127 and 140). Be- 

 tween 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 umbilical cord (Figs. 122 and 123). At the bend of the 

 intestinal loop is the slender yolk stalk. The cephalic limb of the intestine 

 lies to the right, owing to the rotation of the loop. The small intestine 

 extends as far as a slight enlargement of the caudal limb of the loop, the 

 anlage of the cacum, or blind 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 cavity of the rectum 

 is almost occluded by epithelial cells (Lewis). 



Urogenital System/ The mesonephros is much larger and more 

 highly differentiated 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, lies just mesial to the 

 umbilical 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. Sur- 

 rounding the pelvis is a layer of condensed mesenchyma, or nephrogenic 

 tissue, which is the anlage of the remainder of the kidney. 



Blood Vascular System.- The Heart. In Fig. 125 the cardiac cham- 

 bers of the right side are opened. The septum primum between the atria 

 is perforated dorsad and cephalad by the foramen ovale. The inferior 

 vena cava is seen opening into the sinus venosus, which in turn communicates 

 with the right atrium through a sagittal slit guarded by the right and left 

 valves of the sinus venosus. The right valve is the higher and its dorsal 

 half is cut away. The valves were united cephalad as the septum spurium. 



