CENTROSOME AND SPHERE IN THE EGG OF 



UNIO. 



FRANK R. LILLIE. 



IN this paper I shall present some observations on the rela- 

 tion of the centrosome to the sphere, and of both to the cyto- 

 plasm. It is still undecided, I think, what is to be called 

 centrosome and what sphere in all cases (e.g., see MacFarland, 1 

 McMurrich, 2 and Haecker 3 ) ; and it is still possible to take 

 either side in the controversy as to whether the centrosome is 

 a unique and permanent organ of the cell or not. If the 

 centrosome is not a permanent organ, it must, at some phase 

 in the life cycle of the organism, lose its identity as centrosome 

 and be replaced. Hence, sufficiently exhaustive observation 

 must reveal lack of continuity. If it can also be shown 

 that centrosomes may arise at various points in the cell, the 

 centrosome loses its place as a unique and permanent organ 

 of the cell. If, finally, any of the products of division of the 

 centrosome become other formed elements of the cell, the same 

 conclusion follows. It is this last line of argument on which 

 I shall lay most weight in this paper. 



In the metaphase of tJic first maturation spindle both asters 

 possess the following structure (Figs, i and 2} : 



a. In the exact center is a minute black speck, the centrosome, 

 either round or dumb-bell shaped, into which are "inserted " the 

 central ends of some, at least, of the rays. 4 



b. There are two concentric spheres, corresponding to the 

 "medullary" and "cortical" zones of Van Beneden, bounded 

 by microsomes regularly arranged on the rays. The ground 



1 MacFarland, Dr. F. M., " Cellulare Studien an Mollusken-Eiern," Zool. 

 fa/irbik/u'!-, Abt/i. fiir. Anat. und Ontogenie dcr Thicrc. Bd. x, Heft 2. 1897. 



2 McMurrich, J. Playfair, " The Yolk-lobe and the Centrosome of Fulgur 

 Carica," Anat. Anz. Bd. xii, Nr. 23. 1896. 



3 Haecker, V., " Ueber den heutigen Stand der Centrosomafrage," Verhand- 

 liingen dcr Deutschen Zool. Ges. sit Miinchcn. 1894. 



* The sections, 5 fj. thick, were stained in Heidenhain's iron haematoxylin and 

 Bordeaux red. 



