526 
CLIFFORD DOBELL. 
figs. 11, 12). Their central granules thus appear as short 
rods, sometimes slightly knobbed at the ends. 1 The length 
of these “rods” is different in different individuals (cf. for 
instance, PI. 27, fig. 11, with PI. 27, fig. 12). 
A definite spindle now begins to make its appearance 
between the centroplasts. At first (PI. 27, fig. 12) a few 
spindle-fibres only can be seen, stretching from the centro- 
plasts to the nucleus. They gradually become more con- 
spicuous and numerous, however, until a perfect spindle 
figure is produced, with the centroplasts at its poles — exactly 
like typical centrosomes (PI. 27, figs. 13, 14). Whilst the 
spindle is forming, the nucleus undergoes further changes. 
The karyosome disappears ; ‘the chromatin granules become 
fainter, and chromosomes make their appearance arranged 
transversely across the nucleus (PI. 27, figs. 12, 13). Mean- 
while the nuclear membrane becomes less distinct, and the 
nucleus becomes drawn out towards the poles of the spindle, 
as though it were actively pulled by the spindle-fibres of the 
centroplasts (PI. 27, figs. 12, 13). Finally, the nuclear mem- 
brane vanishes, the chromosomes take up their position on 
the equatorial plate, and a typical mitotic figure results 
(PI. 27, fig. 14). 
There are several details which I have not been able to 
make out with certainty, chiefly on account of the very small 
size of all the structures concerned. In the first place, the 
mode of origin of the chromosomes in the nucleus is extremely 
difficult to ascertain. The appearances are as I have just 
described them, and the following seems the most plausible 
interpretation. The chromosomes are formed chiefly (or 
wholly) from the substance of the karyosome— as in certain 
amoebae and other Protozoa (see, for example, Dobell (1914), 
Jameson (1914), etc.). The “ chromatin ” granules surround- 
ing the karyosome in the resting nucleus probably disinte- 
1 It is probable, I think, that this appearance should be interpreted, 
not as a rod with knobbed ends, but as an optical section of a disc with 
a thickened rim, viewed edgewise. The structure is too small, however, 
for me to speak with certainty. 
