494 
Journal of Agricultural Research 
Vol. I, No. 6 
uniformly smaller cells and less tendency to chain formation than broth 
or agar. The cells at a are from culture lo on milk, b on broth, and c on 
agar, all incubated 48 hours at 37 0 C. The difference between the dis¬ 
tinctly rod-shaped cell found on agar and the small round cell obtained 
from milk is marked. That differences in size of cells are not due entirely 
to differences in the medium is shown by the chain at h. This com¬ 
bination of small and large cells in a single chain is not unusual in broth, 
a medium in which there is a marked tendency to form enlarged and 
abnormal cells. In some cultures the transition from normal cells to 
those of monstrous size and form was so rapid that it was difficult to 
obtain preparations 
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f 
a 
showing what could be 
considered normal cells. 
The most satisfactory 
preparations were ob¬ 
tained in incubating 
broth cultures until a 
distinct cloudiness was 
obtained, centrifuging 
the culture, siphoning 
off the broth, and wash¬ 
ing the sediment with 
sterile water. After cen¬ 
trifuging again the wa¬ 
ter was siphoned off, and 
apreparationmadefrom 
the sediment. This gave 
a clear field suitable for 
examination under a 
high-power microscope. 
Various types of cells which were found in this collection are shown in 
figure 2. It will be observed that much of the variation in these types is 
in size only or in chain formation. The slender-pointed cells shown at F 
were peculiar to the cultures obtained from the mouth of animals, but the 
cultures from this source were not confined to this type. In Table II the 
letter under the heading “Morphology” refers to figure 2, although it is 
obvious that in many cases the assignment to a particular type can be 
only an approximation. The variation of the morphology is so great and 
so easily affected by the environment that it was not considered in the 
final arrangement of groups. It should be stated, however, that among 
the udder cultures the tendency to chain formation was much more 
marked and more constant than among all other cultures. 
METHODS OF DIFFERENTIATION 
When morphological distinctions are lacking, we are forced to use the 
physiology of the organism as a basis of classification. No single system 
Fig. i.—C ells of streptococci, showing variation in size and morph¬ 
ology. a, culture lo on milk; 6, culture lo on broth; c, culture lo on 
agar; d, culture li on lactose bile; e, culture li on broth; /, culture qm 
on milk; g andMt, culture gin on broth. 
