Nuclear Division 103 



existing nuclei. Better methods of investigation rendered possible 

 a deeper insight into the phenomena accompanying cell and nuclear 

 divisions and at the same time disclosed the existence of remarkable 

 structures. The work of O. Butschli, O. Hertwig, W. Flemming, 

 H. Fol and of the author of this article \ have furnished conclusive 

 evidence in favour of these facts. It was found that when the 

 reticular framework of a nucleus prepares to divide, it separates into 

 single segments. These then become thicker and denser, taking up 

 with avidity certain stains, Avhich are used as aids to investigation, 

 and finally form longer or shorter, variously bent, rodlets of uniform 

 thickness. In these organs which, on account of their special 

 property of absorbing certain stains, were styled Chromosomes^ 

 there may usually be recognised a separation into thicker and thinner 

 discs ; the former are often termed Chromomeres^ In the course 

 of division of the nucleus, the single rows of chromomeres in the 

 chromosomes are doubled and this produces a band-like flattening 

 and leads to the longitudinal splitting by which each chromosome 

 is divided into two exactly equal halves. The nuclear membrane 

 then disappears and fibrillar cell-plasma or cytoplasm invades the 

 nuclear area. In animal cells these fibrillae in the cytoplasm centre 

 on definite bodies*, which it is customary to speak of as Centro- 

 somes. Radiating lines in the adjacent cell-plasma suggest that these 

 bodies constitute centres of force. The cells of the higher plants 

 do not possess such individualised centres ; they have probably 

 disappeared in the course of phylogenetic development : in spite 

 of this, however, in the nuclear division-figures the fibrillae of the 

 cell-plasma are seen to radiate from two opposite poles. In both 

 animal and plant cells a fibrillar bipolar spindle is formed, the fibrillae 

 of which grasp the longitudinally divided chromosomes from two 

 opposite sides and arrange them on the equatorial plane of the 

 spindle as the so-called nuclear or equatorial plate. Each half- 

 chromosome is connected with one of the spindle poles only and is 

 then drawn towards that pole^ 



The formation of the daughter-nuclei is then effected. The 

 changes which the daughter-chromosomes undergo in the process 

 of producing the daughter-nuclei repeat in the reverse order the 

 ciianges which they went through in the course of their pro- 



' For further reference to literature, see my article on " Die Ontogenie der Zelle seit 

 1875," in the Progreasus Rei Botanicae, Vol. i. p. 1, Jena, 1907. 



■* By W. Waldeyer in 1888. 



» DiBCovered by W. Pfitznor in 1880. 



* Their eiifltence and their multiplication by fission were demonstrated by E. van 

 Beneden and Th. Boveri in 1887. 



' These important factri, suspected by W. FlemminR in 1882, were demonstrated by 

 E. Heuser, L. Ouignard, E. van Beneden, M. Nusobaum, and C. IIM. 



