METHOD!^ FOR MICROSCOPIC STUDY OF BACTERIA TM 
the liands, to distribute the organisms thoroughly, and poured indi- 
vidually into the sterile Petri dishes. The niedimn is flowed uniformly 
over the bottom of the dish and set aside to harden. 
It will be seen that the first tube inoculated contains the greatest 
number of organisms, and that the third tube would theoretically 
contain but few. The colonies in one of the plates will be so widely 
separated that they can be "fished" with the platinum wire without 
the danger of touching other colonies, and transferred to fresh, sterile 
media. The success of this procedure depends largely upon a rigor- 
ous observance of details. The mouths of the tubes and the cotton 
plugs should be flamed thoroughly before inoculation is practised, and 
the transfer of the contents of the tube to the Petri dish must be done 
carefully to prevent contamination. The cover of the Petri dish 
should be raised with the left hand, l)ut directly over the bottom, to 
prevent the entrance of ad\'entitious bacteria from the air. The 
mouth of the tube should not touch the bottom or edge of the Petri 
dish and, fin'ally, the cover of the latter should be replaced at once. 
After the medium has hardened the plates are incubated — gelatin 
plates at 20° C., agar plates at 37° C. It is customary to invert agar 
plates during incubation; when agar cools and becomes solid a con- 
traction takes place which squeezes out some fluid. (This is well 
defined in slanted agar as the water of condensation.) If the fluid 
were allowed to remain on the surface of the agar plate it would 
convert the surface potentially into a broth culture, in which the 
various organisms would mix in hopeless confusion. Inversion of 
the plates pre^'ents the accumulation of moisture on the surface to 
a large degree; the water of conflensation collects on the cover instead. 
The porous tops recommended by Hill may advantageously be used — 
they absorb moisture as it is formed. Gelatin plates are not inverted; 
fluid is not expressed as the medium solidifies, and liquefied gelatin 
formed during the growth of actively proteolytic organisms would 
collect on the cover and probably contaminate the entire plate. 
2. Streak Method.— The isolation of pure cultures of bacteria by 
the streak method differs from the plate method in that the medium 
(gelatin, agar, blood serum) is not inoculated in the fluid state; the 
necessary dilution to secure isolated colonies is attained by drawing 
a platinum needle infected with bacteria several times across the 
surface of sterile, slanted gelatin, agar, blood serum, blood-agar or 
other solid medium, each time covering an area not previously touched. 
E\'entually a degree of dilution is reached where discrete colonies are 
discernible. Recently \^arney^ has introduced the "spiral streak 
method" for plating bacteria. The method is applicable to aerobic or 
anaerobic types. The principle involved is to draw a charged inocu- 
lating needle slowly from the periphery to the center of a Petri dish 
containing sterile, hardened agar. The Petri dish is revolved during 
' Juur. Infer. Dis., 1927, 41, 190, 
