No. 2 .] COMPARATIVE CYTOLOGICAL STUDIES. 
537 
Foraminifera “ durch Zusammenfliessen anfanglich leicht fliis- 
siger, dann zahfliissiger und schliesslich erstarrender Massen 
entstanden sind.” It may be asked : Why does the nucleolus 
persist through the whole resting state of the nucleus if it be 
not an organ ? It may be simply stored in the nucleus until at 
the time of mitosis, when the nuclear membrane disappears, 
it has an opportunity to leave the nucleus. The only observa¬ 
tions which would prove that the nucleolar substance may 
functionate as an independent organ are those according to 
which the nucleolus contains a contractile vacuole, and thus 
rhythmically contract and expand ; in these cases the nucleolus 
might be regarded as a pulsating excretory organ of the nucleus. 
The hypothesis might be suggested that though the nucleolus 
probably consists of substances which stand in some relation to 
the nutritive processes of the nucleus, and so at the time of its 
first formation may be a functionless, inert mass of substance, 
yet it may at later periods in the history of the resting nucleus 
acquire some active function and thus gradually come to 
acquire the value of a nuclear organ ; this hypothesis is put 
forward merely as a tentative one. According to this view the 
nucleolus might be considered as an organ which serves to 
accumulate in itself the waste products of the nucleus, thus 
serving as a reservoir for such substances ; or it might be con¬ 
sidered as an organ of excretion, to discharge waste products 
out of the nucleus : in either case the nucleolus would seem to 
stand in direct connection with the nutritive substances and 
forces of the nucleus. 
13. Comparison of the Nucleoli in Plants , Protozoa , and 
Metazoa. 
I have made no morphological studies on the nucleoli of 
plant cells, but would judge from the results of botanical inves* 
tigators that they are probably strictly comparable to the 
nucleoli of the metazoan cells. 
Rhumbler (’93) doubts whether the nucleoli of the Metazoa 
and the “ Binnenkorper ” of the Protozoa are homologous 
structures ; and, indeed, there are certain nucleolar structures 
