THE MESONEPHROS ,' I97 



appear in embryos of 1.7 mm., with nine or ten primitive segments (Felix, in Keibel 

 and Mall, vol. 2); in 2.5 mm. embryos (23 segments) all the tubules have devel- 

 oped 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 pro- 

 nephric tubules soon degenerate, but the primary excretory ducts persist and be- 

 come 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 possessing an inter- 

 nal glomerulus alone they differ from the pronephric tubules in that the nephro- 

 tomes are transitory, never opening into the mesonephric chamber. The meso- 

 nephric 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, how- 

 ever, are forined from distinct intermediate cell masses, for caudal to the tenth 

 pair of segments this mesoderm constitutes 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 

 coelom. Thus there is produced on either side of the dorsal mesentery a longi- 

 tudinal urogenital fold, which may extend from the sixth cervical to the third 

 lumbar segment (Fig. 220). Later, this 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 13th, 

 14th and 15th 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. 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 tubule enlarges, becomes thin walled, 



