THE URINOGENITAL SYSTEM. 93 
urinogenital sinus. This development reaches its extreme in the 
higher mammalia, where the urinogenital sinus is completely 
separated from the digestive tube, and where the urinary ducts are 
also transferred from a posterior or hypocystic position on the 
wall of the urinogenital sinus to an anterior or epicystic position 
on the dorsal wall of the bladder. 
The chief organs of the urinary system are the kidneys. They 
are paired organs, lying against the dorsal abdominal wall, approxi- 
mately in the position of the embryonic inter- 
mediate cell mass from which they are formed. 
That of the left side is displaced backward, out of the position | 
of symmetry, on account of the posterior 
development of the greater curvature 
of the stomach. The kidneys appear 
as solid organs, brownish in colour and 
bean-like in general shape, enclosed by 
a fibrous coat, and connected medially 
with the expanded end of the ureter. 
In the rabbit the kidney appears as an 
almost continuous mass, in which, how- 
ever, slight traces of lobulation can be 
distinguished. In many mammals, 
such as sheep and bear, the organ is 
THE KIDNEYS. 

Fic. 50. The left kidney, divic 
composed of distinct and separable ed horizontally lengthwise, cut sur- 
face of dorsal half. c, cortical sub- 
Iopiles «his condition isiclearly shown. Sence, ty medullany, ‘substance; 
i x : p, renal papilla; u, ureter. 
in the human kidney in foetal life, and 
though much more concentrated in the adult, the lobulated con- 
dition appears internally in the division 
of the ureter into several renal calyces, 
each of them connected with a corresponding renal papilla. 
When horizontally divided (Fig. 50), the kidney is seen to be 
made up of a more vascular and granular external layer, termed the 
cortex, and of a somewhat radially striated, central mass, termed 
the medulla. In the rabbit, there is a single renal papilla, and 
the expended end or pelvis of the ureter is undivided. Notwith- 
standing the solid appearance of cortex and medulla, the kidney is 
made up of a system of tubules, the relation of which to the vascular 
system and to the outside of the body is such that fluid materials 
FORM IN MAMMALS. 
