344 TEXT-BOOK OF BACTERIOLOGY. 



They are readily accessible to staining- with anilin colors, espe- 

 cially by Gram's method. 



The development of the colony on the gelatin plate is nearly like 

 that of the erysipelas bacilli, but their growth is not so dense, for 

 they reach a greater extent very much sooner and cover a 

 greater region of the medium, thus imparting to the colony an es- 

 pecially delicate transparent appearance. 



This difference is, perhaps, still more perceptible in the test-tube. 

 With the erysipelas bacilli, we find a dense culture restricted to 

 the vicinity of the inoculation puncture, while here the bluish -gra3 r , 

 dim clouds traverse nearly the whole gelatin from the beginning. 

 This peculiarity is unmistakable, above all in young cultures (up to 

 one week); later on it begins to lose its clearness and is finally 

 lost. 



The bacilli of mouse septicaemia prove, in experiments on animals, 

 infectious for domestic and white mice, pigeons, sparrows, and rab- 

 bits. Chickens, Guinea-pigs, and field-mice are perfectly insuscep- 

 tible, as Koch has especially emphasized. 



When inoculated into the ear of rabbits, they produce an ery- 

 sipelatous inflammation of the subcutaneous tissue, which usually 

 heals and protects the animals against repeated infections. Mice 

 fall sick, just in the same manner as after inoculating- with erysip- 

 elas; the pasty closure of the eyelids is again noticed. 



The post-mortem appearances, the swelling of the spleen, etc., 

 correspond in every respect to the picture we have seen in hog- 

 erysipelas. There is, likewise, the same distribution of the bacilli 

 in the tissue. But the rods of mouse septicaemia seem to occur more 

 copiously in the heart- blood of the animals than those of e^sipe- 

 las, but more sparsely in the lungs. They are very frequently in- 

 closed in cells, either singty or in pairs. 



XXXII. MICROCOCCOCUS TETRAGENUS. 



Koch observed a peculiar kind of bacterium, first in the contents 

 of a tuberculous lung-cavity, afterward repeatedly under similar 

 conditions in the expectorations of patients and in the normal 

 human saliva. It was studied more closely by Gaffky and named 

 " Micrococcus tetragenus." 



They are rather large, perfectly-round cells, forming in culture 

 dense heaps without any particular kind of arrangement. But 

 they look very differently when developed in living tissue and taken 

 from the animal body. 



Four single cells are seen inclosed in a thick gelatinous cap- 



