244 



HISTOLOGY. 



thetic system, which, entering at the root, accompanies the bronchi and 

 vessels; to them it is chiefly distributed. Small ganglia are found within 

 it. The vagus also sends important branches to the lung, which mingle 

 with the perivascular and peribronchial nerves. They contain both 

 medullated and non-medullated fibers. 



URINARY ORGANS. 

 WOLFFIAN BODY. 



The Wolffian body or mesonephros is the "kidney" of adult amphibia, 

 and of certain fishes. It is one of the largest organs found in the human 

 embryo of the second month, but subsequently 

 its renal functions are performed by another 

 structure of later development, the kidney (meta- 

 nephros). As the Wolffian body degenerates it 

 becomes transformed in the male into the ductus 

 deferens and the epididymis, essential portions of 

 the genital tract. Some vestigial remnants 

 may produce pathological growths. In the 

 female the entire organ is vestigial, with patho- 

 logical possibilities. During its development and 

 regression the Wolffian body is a controlling fac- 

 tor in the arrangement of the large veins of the 

 abdomen. 



In an embryo of 35 days (Fig. 275) the Wolff- 

 ian bodies are seen as a pair of long, rounded 

 elevations, one on either side of the root of the 

 mesentery. They extend the length of the ab- 

 dominal cavity and each empties through its Wolff- 

 ian duct into the allantois (described on p. 193). 



The excretion of the Wolffian bodies accumulates in the allantois, which in 

 man is a slender but very long tube. In the pig at a certain stage, it is 

 an elongated, thin- walled sac many times the size of the entire embryo; 

 the large amount of fluid which it contains is due to an unusual develop- 

 ment of the Wolffian bodies. After the urogenital sinus opens to the 

 exterior, the contents of the allantois may mingle with the amniotic fluid 

 in which the embryo is immersed. 



Development of the Wolffian body. In a previous section (p. 22) 

 the development of the mesoderm has been described to that stage when 

 it presents a series of segments (pro to vertebrae), connected by stalks 

 (nephrotomes} with the layers which line the body cavity. From several 



FIG. 275. DISSECTION OF A 

 HUMAN EMBRYO OP 35 

 DAYS. (After Coste.) 



al., Allantois; 1., lung; St., stom- 

 ach; s. tr., septum trans- 

 versum; u. c., umbilical 

 cord; W. b., Wolffian body; 

 W. d., Wolffian duct. 



