Chromosomes in Pollen Mother-cells . 
325 
regularity (Fig. 26). It should be borne in mind that in Lilium there is no 
diagrammatic regularity in the arrangement of the spirem throughout its 
entire course ; some of its turns follow the nuclear periphery, while others 
traverse the interior. In the interior of the nucleus it is not possible to 
follow accurately the entire thread, but it seems that there are no free ends. 
In this stage, as in Podophyllum, very frequently no definite trace of 
a longitudinal splitting is discernible. The stage of the loose and more 
regular spirem soon undergoes a rearrangement and passes into that of 
the second contraction, which is first indicated by a conspicuous knotting 
or entangling of the spirem, usually near the centre of the nuclear cavity, 
and by a tendency of the thread to form loops, some of which radiate from 
the entangled centre (Fig. 27). At this stage of development, there is 
a marked tendency of the sister halves of the longitudinally split spirem to 
diverge rather widely in longer or shorter stretches of the thread, and, 
in the opinion of the writer, this fact more than any other has led to a mis- 
interpretation of the stages which follow. A careful study of the prepara- 
tions showing this and closely related stages shows that, while the divergence 
of the sister halves is frequent at this stage, it does not seem to be the rule ; 
for cells in which little or no divergence of the longitudinally split halves 
are found side by side with those in which the divarication is greatest. It 
does not seem probable, therefore, that the wide divergence of the halves is 
a rule, or that it represents a stage through which all nuclei pass. How- 
ever, as will be seen, the halves come together again at a later stage and 
become so closely applied that no trace of the longitudinal fission can be 
clearly made out. 
The further advance of this rearrangement, or second contraction, 
is manifested by a larger central entanglement of the spirem in which the 
nucleolus may be included (Fig. 28). A large part of the spirem is arranged 
in loops that tend to radiate from the centre, jwith the head or bend of many 
loops toward the nuclear membrane. All the spirem, however, is not dis- 
posed in the form of loops, but some parts may have a straight course, while 
others consist of short and irregular turns. It will be seen also that the free 
ends of some of the loops will be turned toward the periphery, and that the 
number of loops does not necessarily correspond to the number of chromo- 
somes. In Fig. 28 the longitudinal fission can be seen in certain of the 
loops, but the halves do not diverge, while in other parts of the spirem 
all trace of this fission is obliterated. 
The stages occurring between Fig. 27 and the complete differentiation 
of the chromosomes are those that offered the greatest difficulty to those 
who have worked with this problem, and, as stated in a preliminary com- 
munication of the writer 1 , they represent those steps that were regarded as 
the product of poor fixation at the time of his earlier study. There is now, 
1 Bot. Gaz., loc. cit., xl : ’05. 
Bb 
