172 ALLANTOIN. 



When the close chemical relationship of urea to uric acid is taken 

 into account, the statement that those substances which when introduced 

 into the body of a mammal lead to an increased excretion of urea, when 

 introduced into the organism of birds are converted into uric acid 1 , 

 needs excite no surprise. There is further distinct evidence, already 

 referred to under urea, that the conversion is effected in the liver 2 . 

 We know nothing as yet as to the cause of the slight divergence of 

 metabolism which leads to the preponderating formation of urea in 

 mammals and of uric acid in birds and reptiles. It is certainly not 

 due, as some have supposed, to insufficient oxidation in the latter, since 

 the excretion of uric acid is not increased in mammals by artificial 

 disturbance of the respiratory interchange 3 , and it is exactly in birds 

 that the most active oxidational changes, as shown by their higher 

 temperature, is observed. Bearing in mind how readily uric acid 

 yields urea as one product of its oxidational decomposition, it has been 

 supposed that a good deal more uric acid is formed in the mammalian 

 body than is excreted in the urine. In support of this view it may be 

 pointed out that uric acid when introduced into mammals is largely 

 excreted as urea, and that some of the known products of the artificial 

 oxidation of uric acid are occasionally found in their urine, e.g. oxalic 

 acid, oxaluric acid (hydrated parabanic acid), and allantoin 4 . The 

 latter substance is apparently increased (?) by the administration of uric 

 acid 5 . 



3. Oxaluric acid. C 3 H 4 N 2 O 4 . (Hydrated parabanic acid.) 



Occurs in minute traces in normal urine, from which it is extracted 

 by filtering a large quantity of urine very slowly through a relatively 

 small amount of animal charcoal. The charcoal after being washed 

 with distilled water is extracted with boiling alcohol, to which it yields 

 the oxaluric acid as an ammonium salt. The free acid is a white 

 crystalline powder, not very soluble in water : its alkaline salts are 

 readily soluble 6 . 



4. Allantoin. C 4 H 6 N 4 O 3 . (Diureide of glyoxylic acid.) 



The characteristic constituent of the allantoic fluid, more especially 



1 For literature see Bunge, Physiol. path. Chemistry, p. 341. Horbaczewski, 

 Monatshft. f. Chem. Bd. x. (1889), S. 624. Sitzb. d. Wien. Akad. Bd. xcvm. (1889), 

 3 Abth. S. 301. 



2 See also von Schroder, Arch. f. Physiol. 1880. Suppl.-Bd. S. 113. Ludwig's 

 Festschrift, 1887, S. 98. 



3 Senator, Virchow's Arch. Bd. XLII. (1868), S. 35. 



4 Salkowski u. Leube, Die Lehre vom Harn (1882), S. 100. 



5 Salkowski, Ber. d. d. chem. Gesell. 1876, S. 719, 1878, S. 500. 



6 For details see Hoppe-Seyler, Phys.-path. Anal. 1832, S. 159. Neubauer u. 

 Vogel, Harnanalyse, 1890, S. 239. 



