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C. F, U. MEEK. 
tractility, assuming the spindle to be a differentiated portion 
of the asters, which were said to be radial arrangements of 
protoplasm. According to this theory the fission of the 
centrosome, or insertion point, causes the fibrillse to become 
arranged in two radial and opposing groups, and their con- 
traction results iu division of the cell. This explanation was 
accepted in the following year by Boveri, who affirmed that in 
Ascaris the movement of chromosomes towards the equatorial 
plane, and the subsequent divergence of daughter-rods are 
caused by contraction of fibres that have become attached to 
them. Moreover, in 1889 Kabl upheld the view that the 
achromatic figure is only a re-arrangement of the resting- 
stage meshwork ; whereas Boveri believed that it arises from 
the attraction sphere, or archoplasm, which was said to be 
composed of minute granules surrounding the centrosome 
and held there by its attractive force. 
The first restriction of the contraction theory was made in 
1891 by Hermann, who showed that the central spindle 
elongates during mitosis ; he concluded from this that con- 
tractility in the spindle is confined to the mantle fibres, and 
suggested that the remainder are non-contractile supporting 
filaments forming a path for the chromosomes. The re- 
searches of Solger upon leucocytes, published at this time, 
seemed to corroborate van Beneden’s theory ; for their move- 
ments were attributed by him, and later by Heidenhain and 
Zimmermann, to contraction of permanent astral fibrillae 
radiating throughout the cell. On the other hand, Biitschli 
in 1892 again affirmed that astral rays are not fibres, but 
lamellee forming the walls of alveoli. In the same year 
Strasbnrger, who believed the entire achromatic figure to be 
derived from the cytoplasm, suggested that chromosome 
movement is due to chemotaxy; and Lustig and Galeotti 
showed that an unsymmetrical amphiaster may be the result of 
inequality of the centrosomes, and the cause of unequal dis- 
tribution of chromosomes to daughter-cells. 
In 1894 Heidenhain illustrated his theory of mitosis by a 
model, consisting of rubber bands placed at intervals round a 
