138 



THE STUDY OF SEX AND TEN MILLIMETER PIG EMBRYOS 



cal cord. Between them, in sections cranial to this, the allantoic stalk is located. Here it 

 has opened into the crescentic urogenital sinus. Dorsal to the urogenital sinus (dorsal now 

 being at the bottom of the figure, owing to the curvature of the caudal region) is a section 

 of the rectum, separated from the sinus by a curved prolongation of the ccelom. From the 

 ends of the urogenital sinus, as we trace cephalad in the embryo (downward in the series), 

 are given off the mesonephric ducts. 



Lower limb bud 

 R. umbilical artery 



Caudal limb of intestine 



Mesonephric duct 

 Ureter 



Notochord 



Spinal cord 



FIG. 142. Transverse section of a 10 mm. embryo passing through the lower limb buds at the 

 level of the openings of the ureters into the mesonephric ducts. X 22.5. 



Section through the Mesonephric Ducts at the Opening of the Ureter (Fig. 142). 

 The section cuts through both lower limb buds near their middle. Mesial to their 

 bases are the umbilical arteries, which lie lateral to the mesonephric ducts. From the dorsal 

 wall of the left mesonephric duct is given off the ureter, or duct of the metanephros. Trac- 

 ing the sections down in the series, both ureters appear as minute tubes in transverse 

 section. They soon dilate to form the pelvis of the kidney at the level of Fig. 143. 

 Note the undifferentiated mesenchyme of the lower limb buds and their thickened ectoder- 

 mal tips. 



Mesentery 



R. umbilical artery 

 Vein 



Ccelom 



Anlage of metanephros 

 Notochord 



Spinal cord 



FlG. 143. Transverse section through the anlages of the metanephroi in a 10 mm. pig embryo. 



X 22.5. 



Section through the Metanephroi and Umbilical Arteries (Fig. 143). The section 

 passes caudal to the mesonephric ducts, which curve along the ventral surfaces of the 

 mesonephroi (Fig. 124). The umbilical arteries course lateral to the metanephroi; these 

 consist merely of the thickened epithelium of the pelvis surrounded by a layer of condensed 

 mesenchyma, the nephrogenic tissue. 



