652 Beer. — Studies in Spore Development. III. 
by side in the nucleus. Other segments maintain their integrity as unbroken 
loops or more or less bent filaments for some time longer. 
In Fig. 24 it can be seen that there has been a great increase in 
the thickness and staining power of the segments, partly due to the 
condensation of their substance and partly owing to the absorption of 
chromatic material derived from the nucleolus. 
In Fig. 25 the condensation of the segments has proceeded further, and 
we now see that these structures form unquestionable bivalent chromosomes 
in diakinesis. The passage of one structure to the other has, however, been 
so gradual and continuous that it is impossible to mistake the true relation 
existing between them. Many of the bivalent chromosomes at diakinesis 
are still in the form of unbroken loops, like the longer and more slender 
spireme segments which preceded them ; others have separated across the 
loop, so that their two constituent elements are free from one another and 
lie side by side or in the shape of a cross or in some other similar position. 
No one with an unbiased mind who closely studies the series of steps 
which intervene between the first unfolding of the segmented spireme from 
the synaptic coil and the appearance of the fully formed heterotype chromo- 
somes in diakinesis can be in doubt regarding the manner of origin of these 
chromosomes. 
The facts show in an unmistakable manner that each segment of the 
spireme is composed of two elements (somatic chromosomes) joined end to 
end. These somatic chromosomes are at first long and slender, but they 
gradually become shorter and very much thicker, partly by the condensation 
of their substance and partly by the addition of fresh chromatic material 
derived from the nucleolus. At the same time each pair of somatic chromo- 
somes usually becomes more or less sharply bent across into the form 
of a loop, so that the two chromosomes come to lie side by side or, at any 
rate (in the majority of cases), to approach one another laterally. Sooner or 
later the loops become ruptured at the bend so that the two members 
of each heterotype chromosome are finally independent of one another. 
For a long time during their development these chromosomes can be seen 
to be longitudinally divided, but when their condensation has been completed 
this split again becomes lost to sight (Fig. 25). 
In Fig. 26 I have represented a few of the typical forms which these 
heterotype chromosomes may assume in Equisetum. In a we have a short, 
thick loop formed by the two constituents of the bivalent chromosome ; in 
g and e the form is also that of a loop — but of a much more shallow 
character. In b the two somatic chromosomes have developed end to end 
in a straight line without any signs of curvature. In f the pair of chromo- 
somes are arranged in the form of a cross ; in c they lie side by side, 
but diverge from one another at one extremity ; in d the two curved 
chromosomes also lie side by side, but owing to their curvature both ends 
