125 
Acer negundo, L., and Staphylea trifolia , L. 
in the contracted mass loosens up, it is seen in some cases to be double, or 
to show the longitudinal split as in the Lilies, but this phenomenon is of less 
frequent occurrence here. If the longitudinal fission is constant, it is not 
always recognizable, for in the fully developed and regularly distributed 
spireme the presence of a longitudinal fission is rather the exception 
(Figs. 32, 33). Fig. 32 is a slightly older stage than Fig. 31. The cell has 
well rounded off, and more of the spireme is distributed in the nuclear 
cavity. Fig. 33 represents the stage in which the spireme has reached its 
most uniform distribution. The nucleolus is large and presents a uniform 
structure, although what is usually interpreted as vacuoles may sometimes 
be seen within it. Small bud-like protuberances of the nucleolus are much 
less frequent here than in Acer negundo and some other species of Acer . 
The cytoplasm of the cell presents a rather uniform appearance. It some- 
times happens that the condition of Fig. 33 may by shrinkage appear like 
that of Fig. 32, and thus be taken for a much earlier step in the process. 
Care must be taken, therefore, to distinguish between good and faulty 
fixation in this as well as in other mitotic stages. 
As in A. negundo , there now takes place the rearrangement of the 
spireme, or second contraction, which ushers in the cross-segmentation and 
the formation of the bivalents. Here we have a nearer approach to the 
loops and straight portions of the spireme extending out or radiating from 
a more centrally placed knot or close entanglement of the spireme that is 
found in Lilium. The knot or entanglement (Fig. 34) may or may not 
include the nucleolus, and instead of one such entanglement there may be 
two or more. This figure represents a thick section of a nucleus, of which 
the nucleolus lay in a neighbouring section. At this stage there is much 
twisting and kinking of the thread. As a rule the nucleus presents a much 
more confused picture than Fig. 34. Transverse segmentation has un- 
doubtedly begun, although certain free ends and short pieces shown in the 
figure have been made by the knife in sectioning. Because of the tangled 
condition of the thread and of the number of shorter and longer pieces to 
be observed, this is one of the most difficult steps in the entire mitotic 
process in which to obtain a clear view of all details. The spireme, in many 
cases at least, is more slender than in the stage just preceding, its diameter 
being much thicker in some parts than in others. It seems also a more 
sensitive stage, as shrinkage of the cells is frequent. In fact the entangled, 
twisted, and knotted parts of the thread are so confused that it is extremely 
difficult to make out clearly and definitely all details. 
When segmentation is complete there soon follows a shortening and 
thickening of the chromosomes. The bivalents separate, becoming more 
regularly spaced in the nuclear cavity, and the form of each is clearly 
revealed (Figs. 35, 36, 37). The condition of Fig. 34 is frequently present in 
one or two loculi of an anther, while in the others that of Figs. 36 and 37 
