140 
Charles E. Allen 
Apparently the nuclear conditions in Mnium hornum are similar 
to tliose in Polytrichum juniperinum. Are ns (1907) finds in the androgones 
of both species a large “nucleole”, from whose substance he thinks the 
chromosomes are partly forraed; Wilson (1908) describes in the arche- 
sporial cells of Mnium hornum a large nucleole which “contains almost 
the whole of the chromatin” ; and the same author (1911) finds no chro- 
matin in the nuclear reticulum of the androgones or of the androcyte 
mother cells. His figures indicate that Wilson found the same state of 
affairs also in the androgones and androcyte mother cells of Atrichum 
undulatum. 
If Wilson’s figures and those of the Leeuwen-Reijnvaans correctly 
represent their preparations, it is evident that their technique is quite 
inadequate to the study of cytological detail, and it is not surprising 
that these authors have failed to see many structures, both nuclear and 
cvtoplasmic, which are conspicuous in material that has been properly 
fixed and stained. 
No visible change takes place within the nucleus until after the 
determination of the spindle poles and the appearance of many of the 
spindle fibers — processes alreadv described. The nucleus then increases 
considerably in size (Figs. 19 — 24), until, as before mentioned, its membrane 
comes into contact with the kinoplasmic structures (plates or kinetosomes) 
at the poles and with most or all of the spindle fibers in the interpolar 
region. At the same time, the material of the nuclear reticulum begins 
to show an arrangement into Strands (Figs. 20 — 26), which he mainlv 
in the peripheral part of the nuclear cavity. This rearrangement of the 
chromatin and linin is evidently under way in the cell shown in Figure 19, 
whose nucleus has begun to swell. Sometimes it seems that the pulling out 
into Strands has begun at an even earlier period, but it is impossible to 
recognize with certainty the very first stages of this process because of 
the variable appearances of the reticulum in the resting nucleus. 
The chromatin Strands when first apparent (Figs. 20, 21) are very 
irregulär in thickness and frequently anastomose; one or more nucleolar 
masses are still present in the central part of the nuclear cavity. As the 
Strands become of more uniform diameter (Figs. 22 — 24), fewer anasto- 
moses and fewer free ends are visible; ultimately (Figs. 25, 26) the Strands 
become quite uniform throughout, and there are no anastomoses. Whether 
a single continuous spirem is present, I have not determined with certainty; 
the number of free ends not due to cutting, if such there be, is at any rate 
small. In the cell from which Figure 26 was drawn, the spindle axis was 
approximatelv perpendicular to the surface of the section; hence the 
