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. Biitschli, O. Hertwig, W. Flemming, 
HL. 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, which 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 
changes which they went through in the course of their pro- 
1 For further reference to literature, see my article on ‘‘ Die Ontogenie der Zelle seit 
1875,” in the Progressus Rei Botanicae, Vol. 1. p. 1, Jena, 1907. 
2 By W. Waldeyer in 1888. : 
8 Discovered by W. Pfitzner in 1880. 
4 Their existence and their multiplication by fission were demonstrated by E. van 
Beneden and Th. Boveri in 1887. 
5 These important facts, suspected by W. Flemming in 1882, were demonstrated by 
E. Heuser, L. Guignard, E. van Beneden, M. Nussbaum, and C. Rabl. 
