306 



HISTOLOGY 



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 jlfl -W.b. 



The parietal pleura is a thicker and less elastic layer. Ventrally and 

 below, toward the pleuro-pericardial membrane, it exhibits folds containing 

 fat (plica adiposce); and sometimes it forms vascular elevations resembling 

 synovial villi the pleural mlli. Fat may be found in the pleura elsewhere. 



The nerves of the pleura are derived from the phrenic, sympathetic 

 and vagus nerves. In the parietal pleura typical lamellar corpuscles may 

 be found, together with the smaller variety, known as the Golgi-Mazzoni 

 corpuscles. 



URINARY ORGANS. 



WOLFFIAN BODIES AND WOLFFIAN DUCTS. 



On the twenty-eighth of November, 1759, Caspar Friedrich Wolff, then 

 in his twenty-sixth year, defended a thesis entitled " Theoria generations ," 

 and obtained the degree of doctor of medicine at Halle. In addition to 

 the fundamental principles which this renowned thesis set forth, it included 



an account of the development of the kidneys in 

 chick embryos. From the diffuse substantia 

 cellulosa along the ventral side of the spinal 

 column, beginning on the third day of incuba- 

 tion, Wolff saw two elongated bodies gradually 

 take form, and become the kidneys, each being 

 connected with the cloaca by a ureter. These 

 structures, however, are not the kidneys of the 

 adult, and they are generally known as Wolffian 

 bodies; their ureters are the Wolffian ducts. 

 They are large and important organs in human 

 embryos, as shown in Fig. 302. The true or 

 permanent kidneys of mammals arise later, and 

 the Wolffian bodies degenerate, becoming vesti- 

 gial in the female; in the male, however, they 

 acquire new functions, and are retained as a por- 

 tion of the genital ducts (namely the duct of 

 the epididymis). In the embryo they are renal organs built upon the 

 same plan as the permanent kidneys, and moreover in the fishes and 

 amphibia they are the kidneys of the adult. 



Still another renal organ develops in embryos, anterior to the Wolffian 

 body, and it has been found that the Wolffian duct is primarily the due of 

 this anterior kidney or pronephros; consequently the Wolffian duct is some- 

 times called the pronephric duct. The pronephros is the functional 

 kidney in only the lowest of vertebrates (myxinoids). Singularly it has 

 been found that " the human pronephros is by far the best developed within 

 the groups of mammals" (Felix, in Keibel and Mall's Human Embry- 



FIG. 302. DISSECTION OF A 

 HUMAN EMBRYO OF THIRTY- 

 FIVE DAYS. (After Coste.) 



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

 ach; s. tr., septum trans- 

 versum; u. c., umbilical 

 cord; W. b., Wolffian body; 

 W. d., Wolffian duct. 



