246 PROCEEDINGS OF THE AMERICAN ACADEMY. 



type. He finds centrosomes in the " SchJidelganglien von Petromyzon 

 Planeri." 



The first announcement of the presence of a ceutrosome in the nerve 

 cells of an invertebrate was made by McClure ('96) for the unipolar 

 ganglion cells of Helix. He gives no figures, but the brief description 

 shows a condition corresponding in its principal features to the ceutro- 

 some and sphere described by von Lenhossek. 



AVhen only von Lenhossek's and Dehler's papers had come into my 

 hands, I gave my first account of the giant nerve cells of Clymene 

 (Lewis, '96).* Further study has confirmed most of the statements 

 made in that paper, and an opportunity for wider comparison with the 

 statements of other writers strengthens the opinion then expressed, that 

 this ceutrosome and sphere would be found to be of much more general 

 occurrence in nerve cells than had hitherto been supposed. 



Through the kindness of Mr. Hamaker of the Zoological Department 

 of Harvard University, who has permitted me to see his preparations of 

 the nervous system of Nereis, I am satisfied that he has demonstrated 

 centrosomes in the giant cells of that annelid. 



(2) Function. 



Several facts suggest that this structure is a centre of mechanical 

 activity in the cell : — 



1) The concentric arrangement of the protoplasm around the 

 ceutrosome. 



2) The radiations which extend from the ceutrosome. 



3) The strongly eccentric position of the nucleus. 



4) The frequent flattening of the nuclear membrane on the side 

 turned toward the ceutrosome. 



5) The band of denser protoplasm partially surrounding the sphere, 

 but interrupted by the nucleus. 



I have no evidence that these cells undergo division, and it is pretty 

 generally stated by authorities on nerve structures that ganglion cells 

 after an early embryonic period never divide. Although I have sectioned 

 and studied dozens of worms, I have never found the least indication of 

 cell division in any cells of the nervous system of the adult. 



As before stated, each of these giant cells has a single cell process. 



* [Although a copy of Bueliler's paper had, through the courtesy of the author, 

 reached me on Dec. -lO, 1895, by some mischance it was overlooked, and did not 

 come to the attention of Miss Lewis until her paper ('96) on " Ceutrosome and 

 Sphere," etc. had been published. — E. L. Mark.] 



