198 
Allen . — Nuclear Division in the 
Something of the same differentiation in the spirem may often be made 
out in cells which appear in other respects more normal than those just 
described, and which are, on the whole, much better fixed ; this fact 
is evidence that the appearances described within the spirem are not 
artefact, but that they correspond to an actual distinction between the 
substances of which the thread is composed. This distinction is best 
preserved by a strength of the fixing fluid which is less favourable for the 
fixation of the cell as a whole, and especially of the cytoplasmic structures. 
Many observers, beginning with Balbiani (’76, ’81), have noted in the 
spirems of both plant and animal nuclei a similar differentiation into darkly- 
staining granules or discs and a lighter ground substance. The application 
of Flemming’s (’80) term ‘ chromatin ’ was restricted by Pfitzner (’81) to the 
substance of the darker bodies, the ‘ chromomeres ’ of Fol (’91), which 
E. Zacharias (’82) found to be composed of nuclein ; and Schwarz (’87) 
proposed for the lightly-staining ground substance the name ‘ linin.’ 
Fig. 15 shows, at a higher magnification than the previous figures, 
a portion of the two parallel threads before fusion, at the stage of Figs. 13 
and 13 ; in Fig. 16 is shown the beginning of the fusion of the threads, 
some portions of the two threads being still separate, others showing 
a fusion of the linin, the chromomeres being distinct, and others showing 
the chromomeres also fused. The chromomeres are very irregular in 
outline, each seeming to be made up of a number of smaller granules. 
They vary considerably in size. In general the threads approximate in 
such a way that the chromomeres come together in pairs ; but this is not 
an invariable rule, for in some places there seems to be a chromomere in 
one thread which does not find a mate in the other. All stages in the 
fusion of these bodies in pairs may be found ; often a dumb-bell-shaped 
figure is formed first, but eventually the two fuse into an approximately 
spherical mass, still with an irregular outline and the appearance of being 
composed of a number of smaller granules. Fig. 17 shows the structure of 
the thread in a somewhat older nucleus ; here the threads have fused along 
their whole length, but the chromomeres in some parts are separate, and 
all stages of their fusion may still be observed. It will be noticed that in 
some cases successive chromomeres, especially some of the smaller ones, on 
the same half of the fusion thread are in contact or very close together ; 
and such a series of two or more may be in contact with a similar series 
on the other half- thread. It is quite possible that this appearance of 
a fusion of successive chromomeres is connected with the shortening of the 
thread as a whole, whereby is brought about a combination of the smaller 
bodies into progressively larger ones. 
Fig. 18 (from a nucleus at about the stage of Fig. 19) shows the fusion 
more nearly completed ; but even here there is occasionally a pair of 
chromomeres which have not yet united. I have not found instances of 
