460 Sargant . — The Formation of the 
like character of each segment and the twist of the two 
segments of each chromosome round each other are even 
clearer in tangential sections of nuclei at this stage. Such 
sections often show the structure of one or two chromosomes 
with diagrammatic precision (Fig. 19 a). 
The question to be resolved by the examination of stages 
intermediate between that just described (Fig. 19) and the 
typical spirem (Fig. 14) is of fundamental importance. The 
structure of each ribbon-like segment in the immature chro- 
mosome precisely resembles that of the spirem ribbon itself. 
Suppose the spirem ribbon to have fallen into twelve lengths, 
each of which then doubled on itself, the free ends twisting 
round each other. The transition to such forms of chro- 
mosome as those represented in Figs. 19 and 20 would be 
accomplished by simple contraction of the twisted lengths of 
ribbon, together with a fission at the sharp bend in some 
cases. The fate of the twisted segments can be followed 
with perfect certainty (Figs. 20-24). During karyokinesis 
each pair is separated, untwisting during the process, and thus 
half the segments go to one pole, half to the other. If the 
two segments of each chromosome had been originally one 
long length of spirem ribbon, the karyokinesis separating those 
segments would in fact divide each chromosome transversely. 
The process of formation of twelve chromosomes from the 
continuous spirem ribbon can be followed only in sections 
thick enough to include the whole nucleus. I have therefore 
worked out the earlier stages entirely by means of hand 
sections from alcohol material. Figs. 14-18 are drawn 
from such preparations, and we are at once struck by the 
fact that the spirem ribbon begins to split longitudinally 
while it is still continuous (Fig. 15). It is not separated into 
lengths until this longitudinal fission is complete, and each 
length therefore consists of two distinct filaments .twisted 
round each other and not of a single comparatively broad 
ribbon (Fig. 16). Each filament is necessarily formed of 
a row of chromatin granules connected by a linin thread. 
The double filaments begin at once to contract, and while 
