322 Moltier. — The Development of the Heterotypic 
spirem is formed in the synaptic mass as for the doctrine that a single 
spirem is formed, which then underwent a longitudinal fission. On the 
other hand, when we consider the condition of the chromatin shown in 
Figs. 19, 20, and 21, it is clear that a single spirem, with a single row of 
chromatin granules, is present just prior to synapsis. There is no evidence 
that, in such a nucleus as Fig. 19, two spirems are present. The chromo- 
meres in this case are smaller than those in the post-synaptic spirem, and 
this fact finds its most reasonable explanation in the assumption that the 
chromomeres of the stage of Fig. 19 unite to make the larger chromomeres 
which split longitudinally to form the double row in the spirem. The 
doctrine that the longitudinally split spirem is formed by the union side 
by side of two distinct spirems rests purely upon theoretical considerations, 
and furthermore, when we know, as will be shown in what follows, that the 
two members (chromosomes) forming the bivalent chromosomes are arranged 
end to end in the spirem, all theoretical support for the assumption of 
the lateral fusion of two spirems will have fallen to the ground. The 
explanation remaining, which is most in harmony with all known facts, 
therefore, is that the longitudinal fission in the heterotype mitosis is as real 
as in any somatic division. 
The Formation of the Bivalent Chromosomes from the 
Loose or Hollow Spirem. 
Podophyllum. As stated in a preceding paragraph, the longitudinally 
split spirem, on coming out of synapsis, thickens and shortens to some 
extent, as it takes on the form of a rather loose and regularly arranged 
chromatin thread. With this process almost every trace of the double 
nature disappears, so that, in the absence of any knowledge of a previous 
fission, it would not be possible to say that the spirem is double, since the 
halves are so closely applied as to be indistinguishable (Fig. 8). In cases 
in which the double nature can be seen, the segments are usually twisted 
about each other, as described by the writer in 1897. Between the stage 
of Fig. 8 and the cross segmentation into chromosomes, the spirem under- 
goes comparatively little shortening. It has been assumed by earlier 
observers that the spirem shortens greatly, becoming very much thicker, 
and that the sister threads diverge widely in many places, before the cross 
segmentation takes place ; and such an assumption is necessary under the 
explanation that the halves of the bivalent chromosomes lie side by side 
lengthwise in the spirem. Such, however, is not the case. The spirem 
now tends to become arranged in the form of loops (Fig. 8). This looping 
leads to the second contraction, by which the loops and other parts of the 
spirem that are not looped contract toward the centre of the nucleus in such 
a manner that the loops, the parallel sides of which have become more or less 
closely applied and twisted about each other, tend to radiate from the more 
