KARYOKINESIS. 63 



rative changes. If, however, centrosomes may degenerate in whole classes of the 

 plant kingdom, the centrosome is surely neither so ubiquitous nor so necessar}" a 

 cell organ as the nucleus. 



(2) So far as animals are concerned the centrosome has been found in almost 

 all kinds of metazoan cells, and at nearly every stages of the cell cycle. The 

 historj- of biology shows that the failure to find structures, even by many observers, 

 is no proof that they do not exist, and this is particularly the case with structures 

 so difficult to observe and undergoing so great metamorphoses as the centrosomes. 

 As to the alleged disappearance of the centrosome in the fertilization of tlie egg 

 (Foot, Lillie, Child,), it must be said that this like negative evidence in general is 

 not wholly conclusive. Certainly, so far as my own work goes, I cannot affirm 

 that both egg and sperm centrosomes entirely degenerate, although they do dis- 

 appear, nor can I affirm that the cleavage centrosomes are new formations, although 

 I am unable to trace an}" connection between them and the centrosomes of the egg 

 and sperm. Even if these centrosomes disintegrate, it may be that the new centro- 

 somes arise from some of their fragments ; in fact, such would seem to be the case 

 in Crepidula (see p. 27). The history of the centrosomes in the fertilization is at 

 best a complicated one, and is by no means as clear as in the cleavage of the egg or 

 in the division of tissue cells, and until we have more exact knowledge of the origin 

 of the centrosomes in the fertilization, this doubtful evidence against the continuitv 

 of the centrosomes should not be permitted to outweigh the' positive evidence in 

 favor of their continuity afforded by ordinary mitoses. 



(3) The view that the centrosome is only the meeting point of astral rays or 

 that it represents merely a condensation of the cytoplasm, or that it is an enlarged 

 microsome, entirely neglects to take account of the complex structure and metamor- 

 phoses of the centrosome, as well as of its division and persistence. These are by 

 all odds the most characteristic features of a centrosome, and until it has been shown 

 that the cytoplasmic structui'es mentioned above are capable of reproducing these 

 characteristic features, it may well be doubted whether they are really centrosomes. 

 The mere formation of cytoplasmic radiations is in itself no positive indication of 

 the presence of a centrosome, since such radiations are found in the higher plants 

 where centrosomes are wholly lacking (Ostei'hout, Mottier). in non-living substances 

 such as carbolic acid and chloroform, gelatin and albumen (Roux, Biitschli, Fischer), 

 where there is certainly no centrosome with the characteristics described above ; 

 around mid-bodies (see figs. 60, 61), and in many of the multiple and accessory asters 

 found in cells under normal and artificial conditions, which show no body at the 

 center of the rays (Mead, Lillie, Morgan, et al.). 



(4j The fourth class of facts which speak against tlie theory of the persistency 

 and morphological importance of the centrosome forms by all odds the most serious 

 objection to that theory which has yet been raised ; I refer to the experimental pro- 

 duction of centrosomes both in fertilized and in unfertilized egg cells by the action 

 of various solutions (R. Hertwig. Mead, Morgan, Loeb, Wilson). It may well Ije 

 doubted whether all of these structures are centrosomes, but that some of them 



