22 MORPHOLOGY. 



remain united in chains (Fig. 4, a), they are described as streptococci, 

 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, b). 



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, 6. 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 

 a pair. 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 made in a liquid medium. Some of 



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CD **f 



00 SB 



e 

 Fio. 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 a fresh culture, in which a little material 

 >u>l'Mil.Ml in water is examined under a comparatively low-power 

 objective one-sixth (Fig. G). 



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 a slender rod or elongated filament. But it 

 must he remembered that \ve may h;ive short rods and long filaments 

 in a pure culture of the same bacillus the typhoid bacillus, for 



