94 ANATOMY OF THE RABBIT. 
to be excreted may pass into them from the blood stream. The 
primary tubule (Fig. 51A) begins in each case in the cortical 
substance with a cup-like structure, known as a renal or Mal- 
pighian corpuscle, containing a network or glomerulus of 
minute vessels from the branches of the 
renal artery. The blood stream is thus 
separated only by a thin membrane from 
the cavity of the tubule, the wall of which is complete. The 
terminal parts of the tube system have a characteristic course 
in the kidney substance, which accounts for the difference in 
appearance as between the cortex and 
medulla, and are ultimately connected with 
common collecting tubules opening on 
INTERNAL STRUCTURE 
AND FUNCTION. 
e the surface of the papilla. The excreted 
fluid, urine, contains characteristic nitro- 
genous waste materials, usually urea, but 
in some cases hippuric acid. These mater- 
ials are formed in the liver and perhaps 
m 
elsewhere in the body. 
Like all other parts of the urinogenital 
the structure and embryonic 
development of the mammalian kidney 
affords a remarkable illustration of the 
extent to which the adult form of an organ 
is dependent upon ancestry. In the verte- 
system, 

Fic. 51. Kidneytubules. A, 
plan of arrangement in adult 
mammal: c, cortex; m, medulla; 
gl, vascular glomerulus; - tc, 
proximal convoluted portion of 
tubule; tr, proximal straight por- 
tion of tubule (Henle’s loop); 
tp, collecting tubule to renal 
papilla. From Weber, after v. 
Ebner. B, plan of a single 
primitive kidney tubule in 
lower vertebrates, glomerulus; 
np, nephrostome; cl, coelomic 
epithelium; t, body of tubule; 
d, longitudinal duct. 
brate phylum, three pairs of kidneys have 
been recognized. They occur in antero- 
posterior order in the bedy, either in embryo 
or adult, they are of increasing special- 
ization, and their order of appearance and 
functional value are directly associated with the degree of general 
specialization of the groups in which they occur. 
HOMOLOGIES OF 
VERTEBRATE KIDNEYS, Mesonephros, and metanephros. 
mammals, while the mesonephros is embryonic. 
These organs 
have been designated as pronephros, 
The 
metanephric kidney is characteristic of 
The latter is, 
however, the adult kidney in the frog and allied animals, its duct 
ia the male serving both as reproductive duct and ureter. 
The 
