ORIGIN OF THE MIT OTIC FIGURE fj 



many kinds of adult tissue-cells during their resting state ; for example 

 in pigment-cells, leucocytes, connective tissue-cells, epithelial and 

 endothelial cells, in certain gland-cells and nerve-cells, in the cells 

 of some plant-tissues, and in some of the unicellular plants and ani- 

 mals, such as the diatoms and flagellates and rhizopods. On the other 

 hand, Van Beneden's conception of the attraction-sphere has proved 

 untenable ; for this structure has been clearly shown in some cases 

 to disintegrate and disappear at the close or the beginning of mitosis 1 

 (Fig. 27). 



Whether the centrosome theory can be maintained is still in doubt ; 

 but evidence against it has of late rapidly accumulated. 



In the first place, it has been shown that the primary impulse to 

 cell-division cannot be given by fission of the centrosome, for there are 

 several accurately determined cases in which the chromatin-elements 

 divide independently of the centrosome, and it is now generally agreed 

 that the division of chromatin and centrosome are two^parallel events, 

 the nexus between which still remains undetermined. 2 



Secondly, an increasing number of observers assert the total disap- 

 pearance of the centrosome at the close of mitosis ; while some very 

 convincing observations have been made favouring the view that cen- 

 trosomes may be formed de novo without connection with preexisting 

 ones (pp. 213, 305). 



Thirdly, a large number of recent observers (including Strasburger 

 and many of his pupils) of mitosis in the flowering plants and 

 pteridophytes agree that in these forms no centrosome exists at any 

 stage of mitosis, the centre of the aster being occupied by a vague 

 reticular mass, and the entire achromatic figure arising by the 

 gradual grouping of fibrous cytoplasmic elements (kinoplasm or 

 filar plasm) about the nuclear elements. 3 If we can assume the cor- 

 rectness of these observations, the centrosome-theory must be greatly 

 modified, and the origin of the amphiaster becomes a far more com- 

 plex problem than it appeared under the hypothesis of Van Beneden 

 and Boveri. That such is indeed the case is indicated by nothing 

 more strongly than by Boveri's own remarkable recent experiments 

 on cell-division (referred to at page 108). 



C. DETAILS OF MITOSIS 



Comparative study has shown that almost every detail of the pro- 

 cesses described above is subject to variation in different forms of cells. 

 Before considering some of these modifications it may be well to point 

 out what we are at present justified in regarding as its essential 



1 Cf. p. 323. a Cf. p. 1 08. * C/. p. 82. 



