148 
Dig by. — On the Archesporial and 
which the course of events becomes obscured. Whereas during synapsis 
the association of the halves of univalent chromosomes is completed, so 
during second contraction the conjunction in pairs of entire univalent 
chromosomes is achieved. 
It is not possible to find a nucleus which shows diagrammatically the 
conjunction of all its spireme filaments, but by working through every 
nucleus of a sporangium at this stage, isolated instances may be observed. 
In a single nucleus several pairs of filaments may show conjunction, or per- 
haps only one pair may be conjoining, or there may be no trace of conjunc- 
tion whatsoever. It is only by comparing numbers of nuclei, and by 
checking them with those of pre- and post-stages, that it is possible to arrive 
at a conclusion on the matter. 
When the nucleus is in the late hollow spireme stage, the first indication 
of conjunction may be observed in well-fixed preparations. These early 
stages, so unmistakable in the particular material on which this investiga- 
tion is based, had never been observed in the fixations of former years. 
Moreover, the phases subsequent to second contraction, which seemed 
hitherto to defy interpretation, are here intelligible and clear, and there are 
many graduated transitional phases to be seen. The advantages which 
have made the elucidation of these difficult and crucial stages possible are 
due to the successful fixation carried out in the manner already described 
( P . 136). 
The first sign of conjunction is recognized by the tendency of the sides 
of the loops (i. e. of spireme filaments) to incline towards one another, and 
at the points where they are bent in closely together they become joined by 
very fine connecting strands (PI. IX, Figs. 54 and 55). The surface of the 
spireme is slightly drawn out at the place of attachment of these strands. 
This is the first step in the combination of entire univalent spiremes (fila- 
ments) or chromosomes in pairs to form the bivalent spiremes which will 
become the heterotype chromosomes. In Fig. 55 the sides of the loops are 
connected at one point suggestive of a future U-shaped chromosome; 
Fig. 56 typifies the future X -shaped chromosome; whilst Figs. 57 and 58 
show a looping over of the spireme filament characteristic of other hetero- 
type chromosome figures. 
These studies of early conjunction of univalents (filaments) show degrees 
of fission (i.e. traces of the two original separate threads of which the 
univalent spireme is composed) in each univalent spireme, but from this 
stage onwards to the coming out of second contraction fission is usually 
closed. Fission is never visible in the substance of the filaments when they 
are closely conjoined (PI. X, Figs. 60 and 63), though it may be evident in 
the filaments which are still independent (Figs. 65 and 66). 
Fig. 59 is taken from the same sporangium as Fig. 53, but is in 
a decidedly later stage. In this univalent spireme (Fig. 59) complete 
