128 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



dorsal wall of the duodenum and its larger, tabulated- body grows dorsally and 

 cranially (Figs. 118 and 135). 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 umbilical cord (Figs. 117 and 118). 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 enlarge- 

 ment of the caudal limb of the loop, the anlage of the ccecum, 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. 117. 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 dif- 

 ferentiated than in the 6 mm. embryo (Figs. 115 and 119). Along the middle 

 of its ventro-median surface the genital fold is now more prominent (Fig. 117). 

 In a ventral dissection (Fig. 119) the course of the mesonephric ducts may be 

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

 stalk. 



The metanephros, or permanent kidney anlage, lies just mesial to the 

 umbilical arteries where they leave the aorta (Fig. 118). 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. — The Heart. — In Fig. 120 the cardiac chambers 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. The aortic bulb is divided distally into the 

 aorta and the pulmonary artery, the latter connecting with the fifth pair of aortic 

 arches. Proximally the bulb is undivided. The interventricular septum is 

 complete except for the interventricular foramen, which leads from the left ven- 

 tricle into the aortic side of the bulb. Of the bulbar swellings which divide the 



