62 KARYOKINESIS. 



division (shown not only by observation in many animals, but particularly by 

 Boven's ('97) experiments on echinoderm eggs) ; (3) its characteristic structure and 

 metamorphoses, which in a large number of animals (perhaps in all) can be reduced 

 to a common type. 



These features are of such character and importance that they justly entitle 

 the centrosome to the rank and title of a permanent cell organ (Van Beneden, 

 Boveri). One who has followed the history of the cleavage centrosomes through 

 several cell cycles, who has observed their unfailing persistence, the regular cycle 

 of changes in form and staining reactions which they undergo, their complex struc- 

 ture, their form of division, their parallelism in these and in other respects to the 

 nuclei (see p. 55), can no more doubt that these centrosomes are persistent cell- 

 organs than that nuclei or plastids are. 



On the other hand there are the well known facts (l)-that, according to the 

 best testimony, there are no centrosomes whatever in the higher plants (Strasburger, 

 Osterhout, Mottier, et al.) ; (2) that the persistence of centrosomes has been denied 

 in the tissue cells of some animals, and even in certain stages of the egg, particu- 

 larly during fertilization (Foot, '97, Lillie, '98, Child, '99); (3) that various stages 

 intermediate between centrosomes and microsomes or other cytoplasmic constituents 

 have been described (Burger, Reinke, Watase, Mead, Eismond, Erlanger) which 

 indicate that the centrosome is only a temporary differentiation of the cytoplasm ; 

 (4) that artificial asters and centrosomes may be formed in egg cells by the action 

 of various solutions, and that these may function as normal asters and centrosomes 

 (R. Hertwig, Morgan, Loeb, Wilson). 



The contradiction between these two classes of evidence is so complete, and 

 the phenomena in both classes are apparently so well attested, that one would be 

 inclined to seek refuge in the conclusion that in some cases the centrosomes are 

 persistent cell organs and in others temporary structures, were it not for the fact 

 that this contradicton may concern one and the same object (e. g., the eggs of Echi- 

 noderms and of C lies top terus). 



There is certainly no ground to doubt that in the cleavage of the eggs of many 

 animals the centrosomes ai-e, under normal conditions, absolutely continuous from 

 cell generation to cell generation. Nor is there any possibility of doubting that in 

 certain animals the centrosomes show independent growth and division, and that 

 they pass through certain characteristic metamorphoses in this cycle. The only 

 possible interpretation of these undoubted facts is that, in some cases at least, the 

 centrosome is a cell organ of morphological as well as of physiological significance. 



Is the contrary evidence irreconcilable with these well established facts, and 

 must we, therefore, conclude that the persistence of centrosomes, their growth, meta- 

 morphoses and division have no general morphological significance ? I think not, 



(1) If it ba granted that the centrosomes. are not present at any stage in the 

 cell cycle in the higher plants, this does not necessarily contradict the centrosome 

 theory of Van Beneden and Boveri, since the fact that they are present in the lower 

 plants indicates that their absence in the higher plants must be the result of degene- 



