NUCLEOLAR STRUCTURES OF THE CELL. 165 



Metzner (1894) considers that in the mitoses of the salamander 

 testicle the nucleoli play an important mechanical part, since he 

 considers some of them serve to attach the spindle fibres to the 

 chromosomes. 



In regard to the reappearance of nucleoli in the daughter- 

 nuclei, after mitosis, very little has been ascertained. Some 

 authors consider that the nucleolar substance dissolved within 

 the nucleus may be carried over into the daughter-nuclei by 

 the chromosomes. Others, as, e.g., Zimmermann (with the con- 

 clusion " omnis nucleolus e nucleolo "), consider that in the 

 anaphase all the nucleolar substance in the cytoplasm is taken 

 up again by the daughter-nuclei. But the majority of observers 

 leave the question undecided. My own standpoint inclines to 

 the assumption that the nucleolar substance discharged at the 

 stage of the equatorial plate into the cytoplasm is not again 

 taken into the nuclei. This view is difficult to prove. But 

 from a number of comparative observations it seems to me evi- 

 dent that the nucleolar substance is of no special morphological 

 value, and that it is in all probability not carried over in consecu- 

 tive cell generations. If this assumption be proved correct, 

 it leads to the further conclusion that one reason for the dis- 

 appearance of the nuclear membrane in mitosis may be the 

 necessity for a discharge of those nuclear waste products which 

 are in such a physical state that they cannot pass through the 

 nuclear membrane. In the final stages of their existence the 

 nucleoli may well be considered waste products, and the dis- 

 appearance of the nuclear membrane offers an opportunity for 

 their discharge from the nucleus. Thus in mitosis, while 

 nutritive processes are arrested, free play is given to nuclear 

 excretion, and so the amassing of waste products in the nucleus 

 is prevented. 



6. Relation to Centrosomes. The following writers have 

 assumed a genetic connection between nucleoli and centro- 

 somes : Karsten (Psilotum, the observations combatted by 

 Humphrey) ; Wasielevsky, Sala, Carnoy and Lebrun (As- 

 caris) ; Lavdowsky (various cells of animals and plants) ; Wil- 

 cox (spermatocytes of Caloptenus] ; Balbiani (Spirochond) ; 

 Bremer (blood corpuscles of Reptiles) ; Toyama (spermato- 



