of Bacteria and their Allies by Heterogenesis. 387 
to another without further difficulty. The fungus hypha has the power 
of penetrating the cuticle, but only in young and tender structures; old 
and hardened membranes could only be entered when the cuticle had 
ee injured or when it had gained strength by special saprophytic 
nutrition.” 
These results will prove important as standards for compa- 
rison with others to which I am now about to refer*. We 
have seen that in the case of actual infection by Bacteria 
there is need for the co-operation of many organisms in order 
to bring about, by their secretions, the softening of the wall 
of every single cell that is penetrated, that some time is 
required for the operation, and that the softening produced 
must be considerable before any such penetration is possible. 
It will be seen how very different is the state of things in 
eases which I shall cite as instances of the origin of Bacteria 
and their allies in the tissues of plants and animals by a 
process of heterogenesis. 
The presence of two characteristics, wherever they coexist, 
may be regarded as strongly in favour of the interpretation 
of heterogenesis as against infection, as the following remarks 
will show. 
(1) The means adopted by Bacteria for bringing about the 
penetration of cells are such as are associated with the vital 
processes of adult organisms. So that there is no reason to 
think that invisible or scarcely visible germs of such minute 
organisms would have the power of secreting a cytase sufii- 
cient in amount to bring about that degree of softening of a 
cell-wall which has been found to be a necessary preliminary 
to their penetration. Yet in multitudes of cases it 1s minute 
germs of Bacteria and their allies which may be seen developing 
within cells or tissues. 
(2) Again, the process of infection, as described by Potter, 
is one brought about by active organisms which affix them- 
selves to a cell-wall until it becomes softened, and then succeed, 
by reason of this same activity, in boring their way into the 
cavity of the cell. On the other hand, in very many of the 
cases in which, as I maintain, Bacteria and their allies may 
be presumed to be originating by heterogenesis, what can 
often be seen is this—particles becoming visible in the midst 
of homogeneous protoplasm, such particles being invariably 
motionless, but followed soon by the appearance of definite 
Bacteria or their allies, recognizable as such by their shapes 
and modes of collocation. But these Bacteria or their allies, 
* As well as with the case of assumed migration and infection referred 
to on p. 385, note *, 
