Dr. Burnett on the Renal Organs of the Vertebrata. 375 
true circulatory system distinct by itself, that these structures are 
wanting. The kidneys are organs inseparably connected wit 
an important blood-function, and in the Vertebrata where the 
conditions of organization rest upon an active and widely distrib- 
uted circulation, they possess determinate anatomical and physio- 
logical characteristics, and may therefore form the subject of a 
distinct and complete investigation. 
n this account, I limit myself in the present paper to an ex- 
amination of these organs as observed in the four grand classes 
of Vertebrata: Fishes, Reptiles, Birds, and Mammals,—and by 
the terms Urinary Organs, | mean those both of a transient and 
a permanent nature—the Wolffian bodies of most embryos, as 
well as the true kidneys of all adults. 
I have lately enjoyed excellent opportunities for the study of 
the development and intimate structure of these glandular organs ; 
and their apparent peculiarities, as observed in the four different 
Classes, require the detail I have given, in order to convey a clear 
idea of what may be called the formula of their organization. 
I say formula, for the intimate organic structure of a urinary 
organ wherever observed among the Vertebrata, is invariably the 
Same; the variations being only apparent and extrinsic, and due 
to the many modes of combination of a single individual structure. 
As before mentioned, the Renal organs appear under two forms 
viz.: the Embryonic, or the so-called Wolffian Bodies, and the 
Permanent or true Kidneys. 
I. Wolffian Bodies. 
To the physiologist it is a beautiful and suggestive fact, not 
unfrequently observed in embryological studies, that nature some- 
imes puts up a temporary, provisional structure, for the perform- 
ance of an important function, until the conditions of the general 
Organization shall be so far advanced that there can be formed a 
Permanent organ of a certain type, belonging to the animal as 
Such, and which persists through its entire life. Such a fact is 
Presented by the Wolffian bodies. : 
_ In the higher forms of organization, the blood, directly upon 
i$ active circulation, seems to require some means for the remo- 
val from it of certain effete particles, and to effect this a delicate, 
transient structure is erected, to remain only until a permanent 
She in the shape of a true kidney can be formed. 
These temporary kidneys are found in the embryos of all the 
Vertebrata, excepting the Fishes and the Amphibian Reptiles; 
that is, in the true Reptiles, the Birds, and the Mammals. The 
sth of time they persist is, in general, in an inverse ratio to 
the grade of the animal; in fact, this law of gradation seems so 
Marked, that there would at first seem some ground for the opin- 
ion that in the Amphibia and the Fishes, which have only per- 
