No. 2.] COMPARATIVE CVTOLOGICAL STUDIES. 537 



Foraminifera " durch Zusammenfliessen anfanglich leicht fliis- 

 siger, dann zahflussiger 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 



