Chromosomes in Pollen Mother-cells. 
333 
is no such fission in either somatic or spore mother-cells, and the only con- 
clusion remaining is that the spirem is formed double under all circumstances. 
The writer doubts whether any cytologist is prepared at present to defend 
such a doctrine. 
In Podophyllum, as the chromatin thread becomes arranged into the 
hollow and regular spirem, the longitudinal split, as a rule, disappears from 
sight. Whether the sister threads completely fuse, or become so closely 
applied only as to conceal the double nature,, cannot be stated. The latter 
alternative is not improbable, especially when we remember that we have 
to do with two cords composed of a thick fluid substance of exactly similar 
consistency. The next step is the rearrangement of the spirem which 
results in the so-called second contraction figure, in which much of the 
spirem may appear as radiating loops, while the remainder is in the form 
of an entangled mass near the centre of the nuclear cavity. Cross seg- 
mentation now follows, and the rapid shortening of the segments begins. 
In the formation of the second contraction figure, and from the nature 
of the chromosomes as they become more or less free, it is clear that each 
is formed by the approximation of two parts of the spirem that were 
arranged end-to-end in the thread, and not by two sister segments that 
first separated laterally and then came together again, or united at one end 
to form loops, open or closed links, and so on. In Tradescantia the case is 
equally clear, though certain details are more difficult to follow than in 
Podophyllum. The same process obtains in Lilium Martagon and 
L. candidum , although in these species, especially in L. candidum , 
a factor enters which tends to confuse greatly the regular course 
of events, and which has led the writer, together with many others, into 
error. I refer to the tendency in many cases of the sister threads of the 
longitudinally split spirem to divaricate rather widely for longer or shorter 
intervals prior to, or during, the rearrangement of the spirem into the loops. 
In some cases the divergence of the halves seem to be greater when the 
loops were more regular, but as to this being a rule cannot be stated with 
certainty. It should be remembered that a wide divergence of the sister 
threads does not occur in all cells, and, what is of greatest importance, 
those which do separate, as described, come together again before cross 
segmentation, and become so closely applied as to conceal the double 
nature of the thread. This fact seems to have been entirely overlooked by 
Berghs (’04). When one looks at Berghs’ series of figures (1. c., Figs. i-8) 
it seems that he has made out a case that can scarcely be questioned. But 
he neither describes nor figures the stages showing the second contraction 
or rearrangement, as illustrated by my Figs. 38, 29, and 30. He has 
evidently overlooked the fact that the divaricated halves of the spirem 
come together again before complete cross segmentation. In his (1. c., ’04, 1) 
Figs. 2-4 he has shown only isolated portions of the spirem. In order 
