NOTES ON THE DIRECT DETERMINATION OF UREA 
AND AMMONIA IN PLACENTA TISSUE. E 
By FREDERICK S. HAMMETT. 
(From the Department of Anatomy of Harvard Medical School, Boston.) 
(Received for publication, January 14, 1918.) 
INTRODUCTION. 
Certain questions as to method of analysis have come up dur- 
ing the course of some work on a study of the urea content of 
placenta tissue. From the point of view of routine work the 
method of Sumner (1) comes nearest to the analytical ideal of 
combined accuracy, speed, and minimum of manipulation. In- 
asmuch as the results herein reported have been obtained by 
following his procedure no recapitulation is necessary, any altera- 
tions being considered in detail in the text. 
Urease. 
The application of the urea-splitting enzyme of the soy bean, discov- 
ered by Tacheuchi (2) and intensively studied by Van Slyke and Cullen 
(3, 4), to the determination of urea in blood and tissues was first made by 
Marshall (5). I have used, however, an extract, prepared in Folin’s lab- 
oratory, of the jack bean whose urease content was recorded by Mateer 
and Marshall (6, 7). This extract has proved satisfactory in every way. 
Potassium Carbonate. 
Fiske (8) and Van Slyke and Cullen (3) advocate the use of highly con- 
centrated solutions of potassium carbonate for the liberation of ammonia. 
It is more advantageous, however, with placenta tissue to use the solid 
salt, inasmuch as the volume of the contents of the test-tube is already 
large due to the water used in rinsing down and suspending the pulped 
tissue. Considerable variations in the quantity employed are recorded 
in the literature ranging from 5 (1) to 12 (9) gm. of solid carbonate. Van 
Slyke and Cullen (3) determined that the use of less than 1 gm. of ear- 
bonate for each 2 cc. of solution necessitates a longer aeration. 
I have found that 5 gm. of solid carbonate is amply sufficient 
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