144 
Dig by.— On the A rchesporial and 
is proposed to give a brief introductory outline of the scheme of events 
before proceeding with the detailed description (see Text-fig., Nos. 15-35). 
It has been shown how, in the archesporial divisions, each pair of parallel 
threads in the prophase are the reassociating longitudinal halves of a chromo- 
some which had separated during the preceding telophase. The parallel 
threads concentrate, become closely associated side by side, and thus re- 
organize a chromosome. They again separate on the spindle and each longi- 
tudinal half becomes a daughter chromosome, which will, in its turn, split 
longitudinally in telophase. Parallel threads, homologous to those of the 
archesporial prophases, are found in early heterotype prophase, but their com- 
plete separation as daughter chromosomes is postponed, and is only finally 
achieved on the homotype spindle. The paired lengths of threads of the 
heterotype prophases are balled up in synapsis, and whilst in synapsis the 
lateral association of the halves (threads) is completed and the entire univa- 
lent spireme filament emerges as an apparently continuous thick double 
thread. This post-synaptic spireme corresponds to the univalent spireme of 
the somatic prophases before it has segmented transversely into univalent or 
somatic chromosomes. The univalent spireme of the heterotype division 
also segments transversely ; but before and during second contraction these 
entire univalent segments (filaments) join in pairs to organize a bivalent 
combination, the heterotype chromosome. On the heterotype spindle the 
whole univalent chromosome of each bivalent pair parts from its fellow, and 
one passes to either pole. This is therefore a reduction division, for each 
daughter nucleus will contain half the number of chromosomes typical of 
the somatic divisions. On the homotype spindle the entire univalent 
chromosome splits into the two daughter chromosomes. This is the 
division prepared for by the association in pairs of the half univalent 
spiremes (threads) met with in the presynaptic and synaptic phases. 
Heterotype Prophase to Synapsis. 
The earliest definite prophase of the first meiotic division (Fig. 36) 
shows a more or less coarsely beaded linin reticulum. Some of the fine 
threads of the reticulum may run in pairs and some of the granules may be 
arranged in groups of four. These are the first indications of the re- 
association of the chromosome halves (threads) which separated during the 
preceding telophase. From this stage onwards, there is a progressive and 
decided enlargement of the nucleus, entailing an extension of the clear 
areas devoid of chromatic contents. Simultaneously the reticulum loses its 
reticulate character, it becomes drawn out into beaded threads (Fig. 37), and 
gradually the chromatin of the beads infiltrates the linin, thus causing it to 
stain evenly and to assume more definitely the spireme character (Fig. 38). 
This spireme thread shows a gradual but conspicuous parallel arrangement 
of portions of its lengths (Fig. 39). The pairing becomes more accentuated 
