164 
Digby . — On the Archesporial and 
(Fig. 37, &c.), and accordingly establish the nature of each thread of a pair 
to be that of a half univalent chromosome. 
4. First Meiotic Division . 
Osmunda conforms to the telosynaptic view of the origin of the hetero- 
type chromosomes (see Text-fig., Nos. 15-27). 
The paired spireme threads of the heterotype presynaptic prophases 
are homologous with those of the somatic prophases ; that is to say, each 
thread of a pair is derived from the longitudinal half chromosome of the 
preceding telophase. During the phases leading to synapsis, these halved 
univalent spiremes (threads) become arranged in closely parallel pairs 
(Figs. 40, 41, &c.), and the individual halves of each pair are identical. 
Close association of the two individual halves (threads) of each pair is 
achieved during synapsis. All stages in the preparation for, and in the 
realization of, association are to be found. Those cases are particularly strik- 
ing in which the loops of spireme threads are polarized towards one side 
of the nucleus in synapsis, and association in pairs of the sides of several loops 
(i.e. of threads) is seen to be taking place simultaneously (Figs. 47 and 48). 
The entire filament as it emerges from synapsis is wholly of univalent 
nature ; no unassociated threads or halved filaments remain (Fig. 49). There 
may be a space in the substance of the univalent spireme between the two 
recently associated threads ; this space marks the future line of that delayed 
fission which will take effect on the homotype spindle when the daughter 
univalent chromosomes separate from each other during the homotype 
mitosis. 
After the spireme filament has become unravelled, and is distri- 
buted throughout the nuclear cavity, segments of the entire (i.e. made 
up of the joined halves) univalent spireme filament proceed to conjoin in 
pairs. The first indication of this conjunction is to be seen in the drawing 
together of the sides of loops or lengths of spireme filament, the two being 
joined by a fine transverse strand (Fig. 54, &c.). This conjunction in pairs 
of univalent spiremes (i.e. of filaments) is prepared for, and more or less 
achieved, during the stages leading to second contraction (PI. X, Figs. 68, 
69, &c.), but it is finally consummated during second contraction itself 
(Figs. 71 and 72). In the early conjoining stages, the conjoining filament 
segments describe figures closely resembling the future heterotype chromo- 
somes (Fig. 65). The two individual filament segments of each conjoining 
pair are precisely similar (Fig. 69). 
As the bivalent segments come out of second contraction they proceed 
to disjoin and give rise to the typical heterotype chromosome figures 
(Fig. 74, &c.). 
The two entire univalent chromosomes of each bivalent combination 
separate on the heterotype spindle (PI. XI, Fig. 82). As the univalent 
