13* 



THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



cal cord. Between them, in sections cranial to this, the allanloic 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 che 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. 



Cwlom 



Mesentery 



R. umbilical artery 

 Vein 



melamphros 



Spinal cord 



FIG. 143. Transverse section through the anlages ot 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. 



