198 THE UROGENITAL SYSTEM 



formed the paired primary excretory (pronephric) ducts. The pronephric 

 tubules begin to appear in embryos of 1.7 mm., with nine or ten primitive 

 segments (Felix); in 2.5 mm. embryos (23 segments) all the tubules have 

 developed and the primary excretory duct is nearly complete. In 4.25 

 mm. embryos the duct has reached the wall of the cloaca and soon after 

 fuses with it. The pronephric tubules soon degenerate, but the primary 

 excretory ducts persist and become the ducts of the mesonephroi, or mid- 

 kidneys. 



THE MESONEPHROS 



The mesonephros, like the pronephros, consists essentially of a series of 

 tubules, each of which at one end is related to a knot of blood vessels and 

 at the other end opens into the primary excretory duct. Besides possess- 

 ing an internal glomerulus alone they differ from the pronephric tubules in 

 that the nephrostomes are transitory, never opening into the mesonephric 

 chamber. The mesonephric tubules arise just caudal to the pronephros 

 and from the same general source, that is, the nephrotomes. Only a few 

 of the more cranial tubules, however, are formed from distinct intermediate 

 cell masses, for caudal to the tenth pair of segments this mesoderm con- 

 stitutes unsegmented, paired nephrogenic cords. These may extend 

 caudally as far as the twenty-eighth segment. The primary excretory 

 ducts lie lateral to the nephrogenic cords. 



When the developing mesonephric tubules begin to expand, there is 

 not room for them in the dorsal body wall and as a result this bulges 

 ventrally into the ccelom. Thus, there is produced on either side of the 

 dorsal mesentery a longitudinal urogenital fold, which may extend from 

 the sixth cervical to the third lumbar segment (Fig. 220). Later, his 

 ridge is divided into a lateral mesonephric fold and into a median genital 

 fold, the anlage of the genital gland. 



Differentiation of the Tubules. The nephrogenic cord in 2.5 mm. 

 embryos first divides into spherical masses of cells, the anlages of the 

 mesonephric tubules. Four of these may be formed in a single segment. 

 Appearing first in the i3th, i4th and i$th segments, the anlages of the 

 tubules differentiate both cranially and caudally. In 5.3 mm. embryos 

 the cephalic limit is reached in the sixth cervical segment, and thereafter 

 degeneration begins at the cephalic end (Fig. 207). Hence, the more 

 cranial tubules overlap those of the pronephros. In 7 mm. embryos the 

 caudal limit is reached in the third lumbar segment. 



The spherical anlages of the tubules differentiate in a cranio-caudal 

 direction (Fig. 206). First, vesicles with lumina are formed (4.25 mm.). 

 Next, the vesicles elongate laterally, unite with the primary excretory 

 ducts, and become S-shaped (4.9 mm.). The free, vesicular end of the 



