n the "Progressus Rei Botanicae", Vol. I. page 1, Jena, 1907.), 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
(By W. Waldeyer in 1888.), there may usually be recognised a separation
into thicker and thinner discs; the former are often termed Chromomeres.
(Discovered by W. Pfitzner in 1880.) 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 (Their existence and their
multiplication by fission were demonstrated by E. van Beneden and Th.
Boveri in 1887.), which it is customary to speak of as Centrosomes.
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. (These important
facts, suspected by W. Flemming in 1882, were demonstrated by E. Heuser,
L. Guignard, E. van Beneden, M. Nussbaum, and C. Rabl.)

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 o