THE PROBLEM OF MITOSIS. 
571 
that mantle fibres do not shorten during* mitosis — thus con- 
tractility was denied to every portion of the achromatic 
figure in tui*n. 
During the same year His remarked that elasticity of 
fibres is irreconcilable with the fluid condition of pro- 
toplasm : and showed that the contraction theory must at 
least be modified; for in trout germ-cells the mantle fibres 
are not attached by their ends to the chromosomes, but pass 
over them. He observed no congestion of thickened fibres 
in telophases, as must occur if contractile fibres are present, 
and afiirmed that the problem of mitotic mechanism will not 
be solved until further analysis has been made of the chemical 
influences working in division. Moreover, Ziegler denied 
that mitosis in Beroe can be explained by contraction or 
elongation of fibres. He believed that the ovum consists of 
an inner vesicular yolk mass surrounded by a thin protoplasmic 
layer, and that cell division is caused by a thickening of the 
last-named, arising under centrosome influence in the region 
of the segmentation furrow. This idea of peripheral change 
caused by distant action of the centrosome was, however, 
criticised by Fischel and dismissed as purely hypothetical. 
The crossing of rays was at this time dealt with by 
Biitschli and Rhumbler. The former showed that a figure 
resembling a mitotic spindle can be produced artificially if a 
warm film of gelatin containing air-bubbles is cooled and 
later coagulated with chromic acid. The latter pointed 
out that protoplasmic meshwork is held together by cohesive 
force, and that changes in form of alveoli must depend 
upon this as well as upon the forces that effect division ; 
thus the crossing of i*ays was said to be impossible only 
if the centrosomes exercise an equal and uniform attraction 
upon an uniform cytoplasmic network. This explanation 
was accepted by Hacker. 
In this year Meves published an exhaustive criticism of 
cell-division theories. Rejecting Rhumbler’s explanation as 
inconclusive, he repeated that rays cannot represent lines of 
force because they cross, and pointed out that spindle 
