and of other related Primula Hybrids . 367 
this stage, owing to the degree of progress exhibited in the process of the 
combination of the univalent strands. Some of the thick bivalent portions 
may open out into univalent loops ; in others, the sides of the loops may 
be still apart and only united at intervals, whilst in others the loop may be 
curved like a ring, the future ring-shaped chromosome. Although, perhaps, 
in some cases the univalent homologous segments of a bivalent combination 
may never be intimately joined together, yet, as a rule, at this stage, most 
of the univalent segments are united side by side in pairs forming thick 
strands (Figs. 26 and 27), which always reveal their double character. 
There is no definite second contraction in P . floribunda ; it seems to be 
represented by the close approximation of univalent segments in each 
separate bivalent combination (Figs. 26 and 27). This fact materially 
simplifies the difficulties as regards the evolution of the bivalent chromo- 
some, for so frequently the massing of the segments in the second contraction 
hides the sequence of events. In P . floribmida it is often possible to identify 
the limits of the nine bivalent chromosomes (Fig. 27) at this stage. 
Having accomplished the necessary lateral approximation, the two 
univalent sides then proceed to split apart (Fig. 28). At first this splitting 
may be restricted to certain lengths, leaving places where the two are still 
in contact (Figs. 28, 29, and 30), thus repeating the figures of their first 
association, but very rapidly the splitting extends until the two univalent 
chromosomes are only attached at one end, or at both ends when a ring- 
shaped chromosome results (Fig. 31). At the place where the two chromo- 
somes are in contact there is constantly a chromatic swelling (Fig. 31) like 
that which has been described at the points of union of the loops of the 
spireme. As the split separates the chromosomes, the chromatin of each 
becomes concentrated (Fig. 31). 
The sides of the bivalent chromosomes thicken considerably and their 
staining power intensifies. They are typical and beautifully shaped hetero- 
type chromosomes, and there are usually three or four rings amongst them. 
Radiations from four centres appear in the cytoplasm (Fig. 32), and these 
extend to the nucleus, which by this time has lost its limiting membrane. 
Sometimes the four apices of the spindle can be detected in the microscopic 
field. The quadripolar spindle may persist for some time, but eventually 
it becomes bipolar. The chromosomes, which by this time have consider- 
ably contracted, attach themselves to the spindle. The spindle tapers 
to a point at either end and the fibres are coarse and distinct. Sometimes, 
in badly fixed material, where there has been much contraction, the spindle 
remains entire, showing that it is of so tough a consistency as to be un- 
touched by violent chemical influences. The rigid character of the spindle 
radiations has been described by Farmer (10, p. 475) in Fossombronia. 
Often a laggard chromosome caps one of the spindle poles (Fig. 33), and 
then, belated, joins the other chromosomes on the plate. The equatorial 
