i68 C.-E. A. WINSLOW AND ANNE F. ROGERS 



Cultures were grown for this purpose on agar at 20 for two weeks 

 and compared by inspection. Amount of growth and depth of 

 color were recorded in five arbitrary grades as follows: growth or 

 color production, much better at 20, somewhat better at 20, equal 

 at the two temperatures, somewhat better at 37, and much better 

 at 37. 



Thermal death-points were included in the original plan of our 

 experiments and have now been made on 87 cultures. The process 

 used is to inoculate from three- to five-day-old agar cultures into 

 broth tubes brought to the desired temperature in a water-bath 

 heated electrically by a platinum coil, and to expose them for 10 min- 

 utes. The tubes are then cooled and incubated at 37 for six days. 

 At the end of that time, streaks are inoculated from the broth tubes 

 in order to make sure by characteristic growth that the organisms 

 orginally inoculated are present. Tests are made from 55 up to 

 the point where growth fails. The process is so tedious that we have 

 been unable to complete the work, and must omit this property for 

 the present. The general results so far obtained are as follows: 



THERMAL DEATH-POINTS. 

 NUMBER OF CULTURES KILLED AT VARIOUS TEMPERATURES. 



Pigment formation. The production of color by the bacteria 

 is not only markedly affected by contemporaneous conditions of 

 cultivation, but may be profoundly modified by selective action 

 or by the effect of antecedent environment. First, of the conditions 

 which temporarily affect the production of color, without modify- 

 ing the inherent chromogenic power of the organisms, may be men- 

 tioned the medium, the presence of free oxygen, and the tempera- 

 ture. In some bacteria, media of low nutritive value, like potato 

 and Nahrstoff, appear to favor pigment formation, but with the 

 cocci this is not generally the case. Agar has, on the whole, shown 

 a better development of chromogenesis than any other medium tested. 

 The presence of free oxygen is generally an essential for color pro- 

 duction, stab growths being almost invariably lightly colored. We 

 have found a single exception to this rule in a coccus which produces 



