2. The above theories are examined and applied to the
excretion of urea.
3. Experimental work directed towards determining the
mode of urea excretion is reviewed.
4. The xanthhydrol method - involving precipitation of
urea as insoluble crystals of dixanthyl urea - is
5. Following review of the literature dealing with the
part played by urea in elasmobranch physiology, the
xanthhydrol method is applied to ascertain the
method of urea excretion adopted in the kidney of
these fish. It is suggested that the normal
excretion occurs through the glomerular capsule.
Any urgent necessity for urea elimination is met by
secondary tubular secretion of urea.
6. The method is also applied in the kidney of the frog,
rat, mouse, guinea-pig, cat, dog, monkey, and man.
In these the excretion is found to be solely
7. The discussion concerns the comparative value of
the different methods used; and the mode of
excretion of urea as elicited by the above experiments. It is suggested that in all instances the
glomerular capsule is primarily concerned in the
excretion of urea. The apparent discrepancy
between the kidney of the elasmobranch and that of
the frog and the mammals is attributed to the
different part played by urea in the physiology of
these animals. In the mammals and the frog urea
is utilised as a means of nitrogenous elimination
and is excreted by the glomerulus only; in the
elasmobranch urea, in addition to its excretory
function, is vital for the maintenance of osmotic
balance and to this is attributed the secondary
tubular secretion, which, under certain circumstances, is added to the primary glomerular