STRUCTURE OF CERTAIN CHROMOSOMES 9 
a comma in the lower one. This I have no doubt is nothing but 
an out-of-focus portion of the periaxial spiral coming into view 
from a lower depth, in a somewhat tilted chromosome. 
I think the utmost that can be admitted in the way of 
any hollowness of the axis is that this may possibly possess 
a cortical layer somewhat denser than the rest. But I think 
the appearances are sufficiently accounted for by the periaxial 
spiral. 
On the surface of this otherwise homogeneous cylinder there 
runs a spiral of somewhat denser substance than the rest, 
figs. 3 to 14. This periaxial spiral is evidently some- 
what denser than the rest, because it resists decoloration in 
regressive staining more strongly ; but it is evidently of the 
same composition, for its affinities for stains are the 
same. It is not something separate from the rest of the 
cylinder, but is continuous with it. It is not fittigly described 
as a fibre wound round a core: for there is no space between 
the spiral and the rest of the axis; there is no hint of a dis- 
continuity between the two either in surface views or in section. 
Nor should it be described as a fibre countersunk or partially 
embedded in the axis: for if it were a fibre its section would 
show as a small circle (or other figure) having a definite limit all 
round ; but these spirals only show a definite limit outside 
the general surface of the core; inside, they merge in its 
substance indistinguishably. Vejdovsky’s term of ‘chro- 
monema ’ is a misnomer: the thing is not a fibre, but a rib or 
ridge. It must therefore be taken to be a mere spiral con- 
densation of the cylinder substance. 
It is true that cases such as that shown in the left-hand 
chromosome of fig. 3 are not very infrequent. At the middle 
of the longer limb of this chromosome there is a break ; and 
the spiral is seen to bridge over the gap between the two parts. 
But I take it that that is only because its toughness has enabled 
it to resist where the rest yielded; just as when you break a 
twig you frequently get the two parts hanging together by a 
strip of bark. 
The periaxial spiral sometimes seems to course uninterruptedly 
