MORPHOLOGY OF ACTINOMYCETES 355 



no thicker, in most cases, than the typhoid bacillus and reveals no 

 more internal structure than does that organism. One must keep 

 in mind the minuteness of the diameter of the cell in evaluating re- 

 search on internal structures which various workers have found in 

 the cells. One often has the feeling that the microscope has been 

 used to detect structures which are beyond the resolution power of 

 the instrument. 



This mycelium is branched, and in some species rather twisted and 

 curled. It forms a tangled mass like the mycelium of any of the 

 higher molds. Although the branching has been described as dichoto- 

 mous by some authors, it is not. There is a main or axial filament 

 with lateral branches. In most actinomycetes examined the young 

 hyphae of the mycelium appear to be homogeneous, i.e., undiffer- 

 entiated, whether examined unstained or in fixed and stained prepara- 

 tions. In older parts of the mycelium it frequently becomes thicker 

 and one can distinguish within it granules and vacuoles. In certain 

 forms, especially in old cultures on rich media, these vacuoles may 

 become quite numerous and relatively large. 



There has been some difference of opinion concerning the occur- 

 rence of septa in the mycelium. Drechsler "^ states that septa are 

 present, but that the cells so formed are very long. Most other in- 

 vestigators deny the existence of septa. It must be admitted that 

 with such fine mycelium it is somewhat difficult to determine whether 

 certain rather infrequent interruptions in the continuity of the my- 

 cehum are septa or vacuoles. The frequent occurrence of fragmenta- 

 tion of the mycelium into segments analogous to the arthrospores 

 of organisms like Geotrichiim candidum has been considered evidence 

 of the occurrence of septa in the mycelium. This fragmentation oc- 

 curs in the same manner as the fission of bacteria, i.e., the crosswall 

 develops just preceding the division. This can be seen with patho- 

 genic species quite readily. The presence of septa in those species 

 which do not fragment has not been definitely established; if present, 

 they are certainly not numerous. This formation of arthrospores 

 occurs very early and regularly with some species, and rarely or 

 not at all with others, and was first proposed as a basis for a major 

 subdivision of the group by 0rskov.i^ In general, those species which 

 tend to form pellicles on liquid media tend to undergo mycelial frag- 

 mentation, and those that grow in the bottom of the tube do not, but 

 this is by no means an absolute rule. 



The formation of arthrospores or fragmentation spores has been 

 studied particularly by 0rskov. The fragments frequently occur 

 in a characteristic zigzag arrangement, which is best seen in slide 



