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opinion that this view is in the main correct, but while I formerly 
thought that the growth enzymes partly derived from the gall-insect, 
I now recognize that they belong to the plant only and that the 
animal does not introduce enzymes into if. 
Research material. 
In the free living unicellular organisms morphological differentia- 
tion, joined with cell division, is quite or almost quite absent, which 
much simplifies the ontogenetic development. That in this case the 
properties must be represented just in the same way by specific factors, 
that is by heredity units or MENDELIAN factors, as in the cell protoplasm 
of the higher organisms, is beyond question. Although it would be 
erroneous to admit that the number of characters, and so of the 
heredity units or factors of the unicellular organisms must be small, we 
certainly have to deal here with a simpler case than in the multi- 
cellular. Hence it seemed probable that heredity experiments with the 
former would give some chance better to understand the nature of 
the heredity units in general. 
But not all properties are equally well adapted to such a research. 
To show that some character of a cell corresponds to one or more 
units or MENDELIAN factors, that character must be able to change 
by mutability in such a way that the mutants prove to be here- 
ditary constant races, distinctly different from the original form, for 
the conception of heredity units must also for the unicellulars start 
from the possibility of race formation. 
The character to be studied must further be observable with ease 
and accuracy and it must be possible to cultivate the concerned 
organism in a simple way, so that in few days thousands of in- 
dividuals can be examined and that no doubt is left as to their 
distinction from foreign infections. These requirements are very 
well answered by some pigment- and by the luminous bacteria 
as I repeatedly stated before. *) Especially the phosphorescence of 
the latter I have minutely examined, no character being better 
qualified to show the process of mutability and to enable us more 
quickly and precisely to judge of the vital energy of the culture 
material. Errors in the nutrition are in this way prevented, which 
so easily occur in microbiological experiments, in particular by too 
strong concentration and too alcaline reaction. Besides, the function 
of phosphorescence is not only found in certain luminous bacteria, 
but it is widely spread throughout the natural system and a remarkable 
1) These Proceedings, 21 November 1900 and 9 February 1910, 
