G. S. Graham-Smith 
45 
Many of these small colonies are lens shaped, i.e. rounded in one plane 
and biconvex in the other, and are formed in the following way. By 
slipping post-fission movements a flat more or less rounded colony, such 
as that seen in Plate VIII, Fig. 4, is produced. As growth continues large 
numbers of new bacilli are produced, more especially in the central 
portions, and begin to override those already formed. The continuation 
of this process results in the central parts being ultimately composed 
of many layers and the peripheral parts of fewer layers of bacilli. 
Hence the biconvex shape on side view. 
In the case of the stronger and more rapidly growing bacilli, how¬ 
ever, the newly formed organisms in the early stages of growth tend to 
grow in various planes, and the colony consequently assumes a very 
irregular branched appearance. During the later stages of growth, 
when frequent divisions are occurring in the organisms composing 
the early branches, regular slipping post-fission movements with the 
formation of parallel sti’ands of bacilli are seen. Possibly the earlier 
branches produce line of cleavage or areas of less resistance in the agar. 
The development of a deep colony of this type is illustrated in 
Diagram 13. 
1 J 5 4 5 b l 
Diagram 13. Illustrating the development of an organism of the subtilis 
group in the depth of agar. 
In fluid media cultures of organisms belonging to this group show 
free, single, usually motile individuals, since there is nothing to retain 
the rods in their relationship to one another. Pellicles composed of 
parallel bacilli are however common. 
All the facts hitherto observed tend to confirm the hypothesis that 
in the “loop forming” group the structure of the colonies, superficial and 
