Schuett&—Biochemical Study of Lake Mendota Plankton 601 
substances which is indigestible in this method of determining the 
crude fiber. Volk^ in making a biological survey of the 
plankton of the Elbe Kiver in Germany in the summer of 1904 
considered this nitrogen as a constituent of the indigestible 
‘‘ Chitin und Muskelsubstanz ” of the plankton which he exam¬ 
ined. He boiled the defatted sample under examination with 
strong potassium hydroxide solution and weighed the result¬ 
ing residue, reporting it as ‘^Chitin und Muskelsubstanz” (in¬ 
cluding certain salts.) He apparently made no attempt to 
remove the latter by digestion with dilute acid. 
We determined the amount of nitrogen in the crude fiber 
by the Kjeldahl-Gunning-Arnold method and found that it fell 
within the limits of the nitrogen content reported by various 
investigators working on chitin from various sources. 
The nitrogen found in the crude fiber is as follows: 
Sample 
Nitrogen 
calc, on crude 
fiber 
Nitrogen 
calc, on original 
A . 
5.94% 
0.27% 
B.... 
6.75 
.17 
C ... 
7.47 
.42 
D.... 
7.47 
.42 
E..... 
7.25 
.09 
There are tabulated below for comparison the nitrogen con¬ 
tent of chitin reported in the literature. 
Reported by 
Source 
Nitrogen 
Schmidt (®). 
6.53% 
Schlossberger(*). 
iangusta 
silk worms 
6.50 
Peligot {*) . 
6.15-8.30 
^ Loc. cit. 
*Ann. Chem. Pharrn. 54, 298 (1845). 
3 Ibid., 98, 115-120 (1856). 
^Compt. rend. 47, 1034-1039 (1858). 
