TRANSACTIONS OF SECTION K. 583 
their association into lines forming primitive chromosomes, the development of a 
linin network, and the formation of a definite nuclear membrane was gradually 
brought about the development of the typical nucleus, 
In the three groups of plants the Cyanophycew, bacteria, and yeast fungi it 
is not possible to recognise all the various parts of typical nuclei as in the protozoa. 
In none of them do we find a nuclear membrane, nucleolus, chromosomes, or 
spindle figure, or centrosome. We have nothing very tangible, therefore, to com- 
pare with the typical nucleus of the higher plants, and it is no doubt very largely 
due to this that we have so many contradictory accounts of the nuclear structures 
in these forms. 
At the same time the nuclei of the higher plants pass through stages in their 
division which more nearly approach in their structure the simple forms with 
which we are now concerned. Thus the nuclear membrane and nucleolus dis- 
appear, and the chromatin network becomes condensed into a number of homo- 
geneous rods or granules, the chromosomes, which lie free in the cytoplasm. 
There seems to be no reason why we should not consider the simpler chromatin 
structures in the lower plants in the light of these ontogenetic changes, as we may 
term them, of typical nuclei, in order to obtain some indication of the origin and 
phylogenetic development of the nucleus. 
We may take the colourless Beggiatoa as a starting-point; not that this 
form is the lowest, but because its structure is, on account of its size, more easily 
examined, and because it is connected possibly with the Cyanophycee on the one 
hand and with the ordinary bacteria on the other. From a careful examination 
and comparison of its structure with that of other low forms we obtain the 
following diagram, showing their possible relationships as indicated by their 
cytological structure :— 
Beggiatoa alba. 
(Chromatin granules may or may not be present.) 
\ 
\ 
Beggiatoa roseo-persicina, Chroma- Larger Spirilla with colourless 
tium okenii, &c. cytoplasm. (Few small chromatin 
Many chromatin granules often granules.) 
condensed in the centre of the cell. 
Cytoplasm coloured. 
Forms like Cholera vibrio, B. li- 
cy raed yoo: : neola, B. anthracis, Typhus bacillus, 
Central body with chromatin gra- &e. Colourless. (One or two chro- 
nules surrounded by a peripheral matin granules.) 
coloured protoplasm), ‘ 
2 
Alge. Yeast Fungi. 
With one or two chromatin gra- 
nules associated with a _ nutritive 
vacuole, 
Fungi. 
