5o6 MONTGOMERY. [Vol. XV. 



3. Position of the Nucleolus in the Nucleus. 



Where a single nucleolus is present it almost always lies 

 excentrically, though not against the nuclear membrane. Those 

 cases where it regularly occupies the center of the nucleus must 

 be regarded as exceptional ; thus I am unable to agree with 

 Macfarlane that the nucleolus is either the morphological or 

 the tropic center of the cell. At the time of its origin, and 

 often at the time of mitosis, the nucleolus may be in contact 

 with the nuclear membrane. Where a number of nucleoli are 

 present they may be scattered irregularly through the nucleus, 

 or grouped at one point in it, or be concentrically arranged ; 

 their position is often dependent upon the stage of the develop- 

 ment of the nucleus. Thus in the metanemerteans examined 

 by me they lie at the periphery in the smallest germinal 

 vesicles, then wander towards its center, and finally migrate to 

 the periphery again. ^ 



The nucleoli lie in the nuclear sap, as a rule not in any close 

 connection with the chromatin reticulum. But in those cases 

 where the nucleolus may be unusually large it appears to be 

 suspended by the fibers of this reticulum, but not in such a way 

 that the fibers penetrate into its substance, but become simply 

 wound around its surface ; thus it appears that when the 

 nucleolus increases in size it forces apart the fibers of the 

 nuclear network in such a way that the latter gradually pro- 

 duce a latticework on its surface. In this way the nucleoli 

 may be more or less held in position in the nucleus, but 

 Herrick's observations on the gravitation of the nucleolus 

 show that it is not firmly held by the chromatin fibers. The 

 nucleolus is, as it were, a ball lodged in the branches of a 

 tree, its movements hindered by the intervening branches, but 

 nevertheless not immovable. Various views on the mode of 

 suspension of the nucleolus : Pflucke ('95), Heidenhain ('92), 

 Rosen ('95), Jensen ('83), Zimmermann ('96). Note also its 

 peculiar position in Synapta (Leydig, '52). 



1 For the opinions of other authors, </. the reviews of the papers of Pfliicke 

 ('95), Heidenhain {'92), Rosen ('95), Jensen ('83), Leydig ('52), Zimmermann 

 ('96), Schneider ('91). 



