730 THE AMERICAN NATURALIST. [VOL. XXXIX. 



times be recognized as deeply staining globules (the so called 

 extranuclear nucleoli). 



There is left for our consideration that group of kinoplasmic 

 structures termed centrosomes, centrospheres, and blepharo- 

 plasts which, when accompanied by radiations, are called asters. 

 Some authors regard these structures as homologous and believe 

 them to be present in one form or another as permanent organs 

 of the cell in certain types (see discussion of Ikeno, :O4). 

 Against this view stand the well established facts of an increas- 

 ing list of forms, both animals and plants, in which these struc- 

 tures unquestionably arise de novo at certain periods in the cell's 

 history. To the author this evidence seems insurmountable and 

 he cannot believe that the aster is in itself a permanent organ 

 of the cell. We shall not take up the subjects of relationships 

 here for such discussions have proved of little profit except in 

 special cases where the various types of structure are found in 

 closely related forms or in the same life history, and these have 

 scarcely been studied at all. We know so little about the rela- 

 tionships in the thallophytes, where relationships must be sought 

 if present at all, that a satisfactory treatment of the subject is 

 hardly possible at present. One point seems to have escaped 

 attention in the writings of those who have discussed the cen- 

 trosome problem. The active elements of the asters are not the 

 central structures (centrosomes, centrospheres, or blepharoplasts) 

 but the fibrillae which play such important parts as spindle fibers 

 or cilia. This fibrillar condition of kinoplasm has a fixed place 

 in the cycle of cell division appearing with each mitosis and at 

 the time of cilia formation, but the fibrillae are not permanent 

 structures of the cell. There is some evidence that the centro- 

 somes, centrospheres, and blepharoplasts are merely regions for 

 the development and attachment of these fibrillae and as such 

 may stand as the morphological expression of fibrillae-forming 

 dynamic centers rather than as organs which actually induce the 

 development of fibrillae. 



