136 THE STRUCTURE OF THE FOWL 



generation of the tubules begins on the tenth 

 or eleventh day, but they do not completely 

 disappear until about the time of hatching. 

 It is evident that the mesonephros or Wolffian 

 body functions as an excretory organ until 

 the permanent kidneys are capable of taking 

 over the duty. 



3. The metanephros or permanent kidney 

 has a double mode of formation. The true 

 kidney tissue is formed from two or three 

 somites of the trunk, while the collecting 

 tubules and the definitive ureter are produced 

 by the elongation and branching of a diverti- 

 culum from the posterior part of the Wolffian 

 duct. 



Reproductive Organs. — In the development 

 of the reproductive system there is an in- 

 different stage during which it is impossible 

 to determine whether the embryo is male or 

 female. During this period gonads and repro- 

 ductive ducts are present, but there is no 

 indication of the kind of germ -cells to be 

 produced by the gonads, nor which set of ducts 

 will persist and which degenerate. 



The gonads are produced on the medial side 

 of the Wolffian body, their germ-cells being 

 derived from an area of peritoneal epithelium 



