PHYSIOLOGY: /. LOEB 
441 
The absolute value of Ca + Mg required was not always identical 
for the same concentration of NaCl + KCl in different experiments; 
this was probably due to differences in temperature or in illumination 
or in the condition of the larvae. It was, therefore, necessary to com- 
pare only experiments made simultaneously with the same material. 
As a rule two sets of experiments were made simultaneously for two 
different concentrations of NaCl + KCl; the two concentrations of 
NaCl + KCl were usually chosen in the ratio of 1:2 or 1:4. 
Table I gives some of the results. The figures for CaCl + MgCl2 
indicate the number of cc. f m CaCl2 + MgCl2 required to just allow 
TABLE I 
Minimal Amount op CaCl2 + MgCl2 Required to Allow the Larvae to Swim in 50 cc. 
(NaCl + KCl) OF the Following Concentrations 
NUMBER OF EXPERIMENT 
II. 
III. 
IV. 
VI 
VII. 
CONCENTRATION 
NaCl + KCl 
m/16 
m/8 
m/8 
m/4 
(3/16m 
\3/8m 
m/8 
m/2 
m/4 
m/2 
/ 5/16 m 
\5/8m 
/3/16m 
\6/8m 
OF 3/8 M CaCl2 + MgCl: 
REQUIRED 
0.3 
0.4to0.5 
0.5 
0.9 to 1.0 
0.7 
1.3 
0.5 
1.8 to 1.9 
0.8 to 0.9 
1.6to 1.7 
0.9 
1.7 
0.6 
2.4 
VALUE OF 
%a + K 
^Mg + Ca 
27.8 
37.0 
33.3 
35.1 
35.7 
38.5 
33.3 
36.0 
39.2 
40.3 
46.3 
49.0 
41.7 
41.7 
all the animals to swim and to collect in a dense helio tropic cluster; 
a diminution of this quantity by 0.1 cc. resulted, as a rule, in a diffuse 
and incomplete collection while with a further diminution by 0.1 cc. 
only a small percentage of larvae rose to the suface. The endpoint or 
threshold according to our criterion was therefore usually sufficiently 
sharp to allow different unbiased observers to select the same dish as 
representing the threshold in a series. 
It was necessary to wait four hours or still better a day, before the final 
