THE MESONEPHROS 205 



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 forms 

 a capsule surrounding a glomerulus, at the other end opens into the primary ex- 

 cretory duct. They differ from the pronephric tubules in that they do not open 

 into the ccelom, and as many as four may develop in a single segment. They 

 arise from the mesoderm intermediate between the primitive segments and the 

 lateral mesodermal layers, mesoderm which, in human embryos, is not segmented 

 into nephrotomes caudal to the tenth pair of segments, but constitutes the un- 

 segmented nephrogenic cord on either side. This may extend caudally as far as 

 the twenty-eighth segment. The primary excretory ducts lie lateral to the neph- 

 rogenic cords. When the mesonephric tubules begin to develop and 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 cervi- 

 cal to the third lumbar segment (Fig. 213). 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. In 7 

 mm. embryos the caudal limit is reached in the third lumbar segment and in 

 later stages the caudal end of the mesonephros undergoes degeneration. 



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

 (Fig. 199). First, vesicles with lumina are formed (2.5 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 

 and into this wall grows a knot of arteries to form the glomerulus (embryos of 

 5 to 7 mm.) . The tubule, at first solid, hollows out and is lined with a low colum- 

 nar epithelium. The outer wall of the vesicle about the glomerulus is Bowman's 



