No. 3-] NERVOUS SYSTEM OF CYNTHIA PARTITA. \\\ 



in the resting cell. Miss Lewis believes the structure homolo- 

 gous with the centrosome and sphere in dividing cells.^ 

 McClure thinks the central bodies and disc found in Helix are 

 morphologically equivalent to the centrosomes and sphere com- 

 monly found in other cells. The papers of Buhler and Schaffer 

 I have not seen. 



It is evident that, at least in certain stages of its existence, 

 the centrosome has a mechanical influence in the cell proto- 

 plasm. As we have seen in young specimens of Cynthia, a 

 condensation of cytoplasm about the central bodies, with the 

 accompanying indentation of the nucleus, is lacking. But in 

 such cells as contained the centrosome, with its sphere and 

 radiations or condensation, a marked mechanical force seems to 

 be excited. This was shown by the excentric position of the 

 nucleus, the flattening or invagination of the nuclear mem- 

 brane on the side toward the sphere, the condensation and 

 concentric arrangement of the cytoplasm about the central 

 body, and the frequently found radiations extending toward the 

 periphery. However, no instances of mitotic division were 

 found. Binucleated nerve cells were seen, and cases where 

 nuclei were so flattened and distorted by the invagination as 

 to be nearly divided into two parts, but in no case anything 

 like mitosis was found. Recent investigation seems to point 

 to the fact that nerve cells, although they may remain for a 

 long time in a so-called embryonic state, i.e., as neuroblasts 

 functionally inactive, never divide as adult cells. No cases of 

 mitotic division of nerve cells have been yet placed on record, 

 so far as known to the author. It would seem, then, that 

 another explanation must be found for the presence of the 

 centrosome in the ganglion cell. 



More recently is advanced the theory that the centrosome 

 may be left over in the cell from its embryonic state to be called 

 forth into activity by seasonal conditions. Such a view was 

 hinted at by Von Lenhosseck in his reference to Meves's paper 



1 In Miss Lewis's second paper, " Studies on the Central and Peripheral Ner- 

 vous Systems of Two Polychaete Annelids," Proc. Amer. Acad. Arts and Sci- 

 ences, vol. xxxiii, No. 14, 1898 (which came too late to be inserted in the biblio- 

 graphical list), she pictures ganglion cells containing two spheres; but she con- 

 cludes that the ganglion cells, after an early embryonic period, never divide. 



