The Morphology and Physiology of the Nucleolus. 14 5 
stage ” of the cell consists of long branched filamentous pseudo- 
podia which are linked up at their extremities with similar 
outgrowths from other chromosomes so as to form the nuclear 
network. Upon this network is arranged the chromatin in minute 
granules. This fundamental structure, consisting of the essential 
chromosome material, and the framework upon which it is sup- 
ported, is believed by many writers to be almost achromatic (4-8) 
in the “ resting cell ” during intense metabolic activity. In the 
network of the nucleus, however, is the nucleolus, which has a 
strong affinity for dyes. 
During the prophase of mitosis, each of the “ amoeboid ” 
chromosomes appears to withdraw its pseudopodia, the chromatin 
granules are brought close together, and at the same time the 
nucleolus disintegrates and the chromosomes show a strong affinity 
for dyes. 
It almost seems as though we have here a case of cause and 
effect — breaking down of nucleolus, and building up of compact 
chromatic chromosomes from the diffused reticular form. A possible 
explanation of this, is that some of the nucleolar material reacts 
with the spread- out chromosomes, in such a way as to cause them 
to contract or condense, and at the same time become more stable 
chemically. We should expect some such chemical change in the 
constitution of the chromosomes during cell division, as it is essen- 
tial that the daughter chromosomes should retain the same chemical 
constitution as the parent ones, and this would be ensured by the 
chromosomes becoming more stable chemically during cell division. 
After cell division, the chromosomes throw off this nucleolar 
substance, spread out to form a reticulum, become less achromatic, 
and less stable chemically, but more active hence the appearance 
of the nucleolus at the same time as the chromosomes begin to 
spread out to form the nuclear network. 
In support of this hypothesis it should be noted — 
(a) That the chromosomes are usually but faintly distinguish- 
able in the actively functioning cell, but the nucleolus 
is most conspicuous, e.g. oocytes of animals such as 
Echinus and Scyllium. 
( b ) That relatively inactive cells, such as those of the germinal 
epithelium of Limnsea , contain no nucleolus, but 
granules of chromatic material, which seems to indicate 
that the chromatin is combined with nucleolar material, 
and is chemically stable, but relatively less active. 
(c) That in binucleate cells where two nuclei are formed ami- 
totically, and both nuclei do not contain a nucleolus, 
the chromosomes are only reformed after fusion of the 
two nuclei, and the chromosomes reappear at the same 
time as the nucleolus disintegrates. This process has 
been observed by Macklin in living cells (38). 
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