144 ™ E STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



genital sinus, as we trace cephalad in the embryo (downward in the series), are given off the 

 mesonephric ducts. 



Section through the Mesonephric Ducts at the Opening of the Ureter (Fig. 137). 

 — The section cuts near the middle through both lower limb buds. 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. Tracing 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. 138. Note the undifferentiated mesen- 

 chyme of the lower limb buds and their thickened ectodermal tips. 



Section through the Metanephroi and Umbilical Arteries (Fig. 138). — The sec- 

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

 mesonephroi (Fig. 119). The umbilical arteries course lateral to the metanephroi which con- 

 sist merely of the thickened epithelium of the pelvis surrounded by a layer of condensed mesen- 

 chyma, the nephrogenic tissue. 



