THE CENTROSOME AND THE BLEPHAROPLAST. 45 



continuity of the centrosomes ; but from the fact that no such organs 

 exist in the higher plants, and that they seem to be wanting in many 

 Thallophyta as well, this view is greatly weakened, if not rendered 

 quite untenable. 



On the zoological side of the question, the recent researches of Wil- 

 son (1901) on eggs of Toxopenustes, which were made to develop 

 parthenogenetically through certain stages by means of chemical 

 stimuli, throw new light upon the subject. In segmenting eggs 

 induced to develop parthenogenetically by means of a treatment with 

 suitable solutions of magnesium chloride, numerous asters (cytasters) 

 often made their appearance in the cytoplasm in addition to the nuclear 

 asters. Similar asters may arise also in non-nucleated fragments of 

 eggs. These " cytasters," just as the segmentation or nuclear asters, 

 may consist of a very distinct centrosome upon which is centered a 

 system of beautiful radiations. The centrosomes divide, and a central 

 spindle is formed between the daughter centrosomes. In fact, the 

 " cytasters " are exactly like the normal cleavage-asters arising in con- 

 nection with the chromatin. As the evidence seems conclusive that 

 the " cytasters" arise de novo, Wilson concludes that centrosomes 

 occurring normally in cells arise also de novo, and that the doctrine 

 of the genetic continuity of the centrosome is untenable. 



It is not known whether anything comparable to these " cytasters" 

 ever occurs in a plant egg-cell, which may be made to develop parthe- 

 nogenetically by artificial means, and consequently we cannot accept 

 the conclusion upon this basis as applicable to plants. There are, 

 however, in plants many well established facts which argue strongly 

 against the view that the centrosome or centrosphere is an organ of 

 morphological rank. 



In 1897, the author made the unqualified statement, to which he 

 still adheres, that centrosomes or centrospheres do not occur in the 

 higher plants, and nearly all research since made along this line has 

 only confirmed this view. We know now that the structures which 

 Guignard so beautifully figured in 1891 for cells of Lilium were the 

 product of preconceived ideas and the misinterpretation of certain 

 facts. There are still a few observers who persist in seeing centro- 

 spheres in the cells of higher plants, in which a score or more of the 

 most competent cytologists, with the aid of the very best methods, 

 have failed to find any such structures. It may be of some interest to 

 note, however, that the centrospheres figured more recently by these 

 observers are not drawn with the old-time diagrammatic distinctness, 

 and it will probably not be long till these structures will not appear 

 at all in figures illustrating karyokinetic phenomena in Allium cepa 

 and species of Lilium. 



