THE ORGANIC CELL 
105 
grouping of the contractile elements in two systems, each 
having its own centre. The presence of these two systems 
brings about cell-division, and actively determines the paths 
of the secondary chromatic asters in opposite directions. An 
important part of the phenomena of Karyokinesis has its 
efficient cause, not in the nucleus, but in the protoplasmic 
body of the cell.’ (See diagram below after van Beneden and 
Xulin, also the diagram of Heidenhain’s model of Mitosis.) 
Th. Boveri shortly afterwards accepted van Beneden’s views, 
and by his own observations did much to support the theory 
Figures of dividing eggs of Ascaris illustrating Van Beneden’s 
theory of Mitosis. (Van Beneden & Iulin.) 
A. Early Anaphase : each chromosome has divided into two. B. Later 
anaphase showing divergence of daughter chromosomes, a.c. Antipodal cone 
of astral rays. c.z. Cortical zone of attraction sphere, i. Interzonal fibres. 
m.z. Medullary zone of attraction sphere, p.c. Principal cone forming one 
half of the contractile spindle, s.e.c. Sub-Equatorial circle to which the 
astral rays are attached. 
of contractility. He demonstrated that, when a chromosome 
splits, each half is connected with rays from the aster on its 
own side, that these rays shorten and thicken as the half 
chromosomes are drawn apart. The rays behave, in fact, 
precisely in a similar manner to muscular fibres, and from a 
careful study of his work it seems impossible to doubt the 
theory of the contractility of the fibrillae. 
This hypothesis of contraction is very clearly brought out 
in models designed by Heidenhain. The model is easily made 
by marking a circle on a flat surface, and attaching at regular 
intervals along the margin of the circle a set of rubber bands, 
which represent the astral rays. The central ends of the 
