Development of the Third Day 2 1 7 



rudimentary structure, appearing after the 

 formation of the Wolffian body, and disap- 

 pearing again almost immediately. 



There are, then, three pairs of excretory 

 organs in the chick : the pronephros or head 

 kidney, which never functions as a kidney, 

 and which almost completely disappears very 

 early in embryonic life ; the mesonephros or 

 Wolffian body, which develops sooner than 

 the pronephros, attains a large size, and func- 

 tions as the kidney during embryonic life ; and 

 the metanephros or permanent kidney, which 

 begins to develop quite early, but does not 

 become functional until after hatching. 



The Wolffian body extends for the greater 

 part of the dorsal region of the embryo chick, 

 as far back as about the thirteenth somite 

 (Fig. 74). In its fully developed condition 

 it consists of a mass of convoluted tubules, 

 opening, at one end, into the Wolffian duct, 

 and expanded, at the other end, into a Mal- 

 pighian body. The tubules of the anterior 

 and posterior ends of the Wolffian body 

 have different methods of development, and 

 hence must be described separately. That 

 part of the Wolffian body which lies anterior 

 to about the sixteenth somite begins as a series 



