) With the resistant organism the appearance of the clear-zone edge was
entirely different in narrow and wide zones. In narrow zones about 3 mm. wide
the edge was indistinct and quite unsuitable for accurate measurement. Many
scattered colonies (some much larger than in the normal growth) were found
within the clear area (Pl. 2, figs. 4 (i) and 5 ) . No sharp subdivision of the edge
into zones was possible, but the areas of scattered colonies were termed
‘ scatter zones ’ to distinguish them from the differently formed ‘ ghost zones ’
of the sensitive organism. In large zones, 10 mm. or more in width, the edge
was sharp and sufficiently well demarcated to be measured to within 0.1 mm.
with accuracy, but it was finely and irregularly crenated, so that measurement
to this degree of accuracy would not be useful in assay work. The growth in this
marginal region was denser than the normal, but was only poorly differentiated
from it (Pl. 2, fig. 4 (ii)). With zones of width about 5-8 mm., a transitional
form of zone-edge was seen.
Jlicroscopic effects on cell and col