22 MORPHOLOGY. 
remain united in chains (Fig. 4, a), they are described as streptococct, 
and are sometimes spoken of as in chaplets or in torula chains. In 
such chains we frequently find the evidence of recent division of the 
cocci, as shown by the grouping of the elements of the chain into 
pairs (Fig. 4, 0). 
When division occurs habitually in two directions, groups of four 
result, which are spoken of as tetrads. This is the distinguishing 
character of the genus Merismopedia. In these groups of four the 
individual cocci are often flattened at the points of contact, as in 
Fig. 5, b. We also find pairs and groups of three in pure cultures of 
species belonging to this genus, as shown in Fig. 5, c. In these, 
transverse division has not yet occurred in one or in both elements of 
apair. This association of micrococci in tetrads seems to be main- 
tained, in some species at least, by the fact that each group of four is 
enclosed in a jelly-like capsule. The extent of this capsule differs in 
the same species under different circumstances; as a rule, it is most 
apparent when a culture has been madein a hquid medium. Some of 
8% 8B an 
8 OO 
eis b 
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Fie. 5. 
the diplococci have a similar capsule. The jelly-like substance does 
not stain well with the aniline colors and is seen as a transparent 
halo around the stained cocci. Some authors (Frankel and Pfeiffer) 
believe that this capsule is formed by the swelling up of the cell 
membrane as a result of the imbibition of water. 
When division occurs in three directions packets of eight or 
more elements are formed. This mode of association characterizes 
the genus Sarcina. The ‘packet form” is best seen in an un- 
stained preparation from afresh culture, in which a little material 
suspended in water is examined under a comparatively low-power 
objective—one-sixth (Fig. 6). 
Among the bacilli there is room for a wider range of morphologi- 
cal characters. They differ not only in dimensions and in modes of 
grouping, but in form. The relation of the transverse to the longi- 
tudinal diameters affords a great variety of forms, varying from a 
short oval element to aslender rod or elongated filament. But it 
must be remembered that we may have short rods and long filaments 
in a pure culture of the same bacillus—the typhoid bacillus, for 
