Multiplication and Migration of Nucleoli in Nerve Cells of Manunals 207 



takes place in the nucleolus when it passes into the cytoplasm. 

 It also seems to ofter a simple and obvious means of judging at once 

 whether a nucleolus has been extruded naturally or otherwise. 



The pseudopodial processes observed in the nuclei of the nerve ganglion 

 cells in adult and young animals seem frequently to be intimately connected 

 with the phenomenon of the migration of the nucleoli. The protrusion of 

 the nuclear membrane described above seems to per.sist in many cases long 

 after the nucleolus has left the neighbourhood of the nucleus. This is 

 particularly noticeable in cases where the nuclear membrane has been 

 depressed in the middle, as already described. In optical section this gives 

 the appearance of two protrusions with the concavity between them directed 

 towards the extruded nucleolus. As to whether these protrusions are 

 always the protrusions formed in connection with the passage of the 

 nucleolus which have persisted in cases where the nucleolus does not 

 remain attached to the nuclear membrane, or whether they represent 

 maybe a separate phenomenon, the present observations do not appear to 

 give any definite suggestion. In any case, the occurrence of the extruded 

 nucleoli and the protrusions in certain definitely relative positions seem 

 to be too frequent to be due to a mere coincidence (figs. 17-19). 



Whatever may be the case with regard to the nerve ganglion cells in 

 embryos, it has been found absolutely impossible to demonstrate centrosomes 

 or astral rays in the material used in the present observations.^ 



After its extrusion from the nucleus, the nucleolus travels towards the 

 periphery of the cytoplasm of the cell (figs. 17, 18, and 19). When it 

 reaches the periphery of the cell it sometimes passes bodily through the 

 surface membrane and is set free among the surrounding cells (fig. 20). 



This is, however, the sequence of events in the case of some onl}' among 

 the nucleoli. In other cases the nucleolus ma}^ be seen Ij'ing on the inside 

 of the surface membrane of the nerve cell in the immediate neighbourhood 

 of a leucocyte or in the case of cerebro-spinal ganglia of the nucleus of a 

 capsular cell. Here the substance of the nucleolus seems to pass piece- 

 meal through several small openings, and to be absorbed into the cytoplasm 

 of the neighbouring cell. The absorbed material seems to lie close to the 

 nucleus of the cell that has absorbed it (figs. 21 and 22). 



Those nucleoli that pass out of the nerve cells without being disintegrated, 

 seem sometimes to be taken bodily into the cytoplasm of a capsular cell or 

 of a leucocyte, where they are probabl}- disintegrated (figs. 23 and 24). 



In studying cerebro-spinal ganglia, while it is quite easj- in very many 

 cases to say definitely whether a particular cell is a leucocyte or a capsular 



^ Tlie term " centrosonie " appears lo liave been used somewhat loosely upon some 

 occasions. The sense in wliich it is used here is that generally accei)ted by oy tologists, i. e. 

 minute structures, oval or bean-shaped, generally two in number, sometimes surroimded 

 by an archoplasm or attraction sphere, which is usually contiguous to the nucleus. We 

 think it possi1)k^ tliat the extruded nucleolus lying upt)n the nuclear membrane, as shown 

 in figs. 14 and 15, may very ]n'obably have been mistaken for the archoplasm, particularly 

 as the nucleolar contents are granular in appearance at this stage. 



VOL. I. — APRIL 1908. 14 



