THE KIDNEY AND URETER. 425 



On the eleventh day a diverticulum arises from each Wolf- 

 fian duct, just before this reaches the cloaca, and grows forwards 

 dorsal to the Wolffian duct. During the twelfth day this 

 diverticulum (Fig. 150, KD) increases in length ; its blind anterior 

 extremity dilates, and gives rise to branching tubular processes ; 

 while the mesoblast surrounding the processes becomes more 

 compact than elsewhere, giving definite shape to the organ. 



The structure formed in this way becomes the kidney, the 

 original diverticulum from the Wolffian duct forming the ureter. 

 The kidney extends forwards, dorsal to the Wolffian body, and 

 overlapping this, so that both the Wolffian body and the kidney 

 may be cut in the same transverse section (Fig. 166, KM and KT). 



In the later stages, the lateral branches from the ureter sub- 

 divide, and elongate considerably to form the kidney tubules. 

 There are for some time no Malpighian bodies in the kidney, 

 but these are ultimately formed in connection with the blind 

 ends of the tubules, in the same manner as in the Wolffian 

 body. 



The kidneys are, from the first, compact bodies ; they early 

 acquire their characteristic shape (Fig. 160, K), and also their 

 asymmetry in position, the right kidney moving some distance 

 further forwards than the left. 



The Wolffian duct and the ureter of each side open, on the 

 twelfth day (Fig. 150, KG, KD), by a common duct into the 

 urino-genital sinus. In the later stages, by unequal growth in 

 different directions, and by absorption of the common duct into 

 the urine-genital sinus, the relations become altered ; the Wolffian 

 ducts still opening into the urino-genital sinus, but the ureters 

 now opening directly into the bladder. 



4. The Miillerian Duct. 



The mode of the development of the Miillerian duct in the 

 rabbit has not been very clearly ascertained. About the twelfth 

 or thirteenth day it is present as a peritoneal funnel, lying to 

 the inner side of the Wolffian body, close to its anterior end ; 

 from the funnel a duct arises which crosses over, dorsal to the 

 Wolffian body, and then runs backwards a short distance, lying 

 very close to the outer side of the Wolffian duct, and ending 

 blindly behind. During the succeeding days the Miillerian 



