148 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



Streptococci form sm^ll white colonies which show no tendency 

 to run together to form a film. The centre of each colony is 

 more opaque than its periphery. 



Staphylococci form a more or less uniform film, the colonies 

 extending laterally and fusing together. The growth is opaque, 

 and is of a dead white, lemon, or orange colour, according to the 

 nature of the staphylococcus present (albus, citreus, or aureus). 



Anthrax bacilli form small white colonies, having the " barrister's 

 wig " appearance already described. 



The colonies of the pneumococcus axe. small flat white points, 

 which do not tend to fuse together. They are difficult to see 

 when they are young, and, in case of doubt, the tube should be 

 returned to the incubator. 



The colonies of the typhoid bacilli and the B. coli axe whitish 

 and opalescent. They usually have an angular or poly- 

 gonal appearance when small, and tend to run together when 

 older if they are thickly set. Their discrimination must be left 

 to an expert. 



The bacillus of plague forms white colonies which are circular or 

 have a crenated outline ; they tend to run together, and form a 

 uniform film over the surface of the medium. 



The gofwcoccus, if it develops, forms very minute transparent 

 colonies which have been compared to droplets of dew. They do 

 not become confluent. This organism will not grow if trans- 

 planted on to the surface of ordinary media, unless a film of blood 

 be previously spread over it. 



After cultures have been obtained they are to be examined 

 microscopically by the method described on p. 22, and the morpho- 

 logical appearances compared with those of the pathogenic 

 organisms which we have enumerated. It is especially important 

 to test whether the organism which has been isolated stains by 

 Gram's method or not. 



ESTIMATION OF THE OPSONIC POWER OF THE 



BLOOD 



A very important branch of blood-work, and one that seems 

 destined to be of great value in the future, has been introduced by 

 Wright, who has demonstrated the presence in the blood of sub- 

 stances which he calls opsonins, and which have the power of 

 acting on pathogenic bacteria and altering them so that they can 



