A critical study of the cytology of Crepis %urens. 
135 
nin parallel to one another and condense to form whole univalent Strands. 
The space between the condensing halves is the line of fission which 
wUl divide the daughter univalent chromosomes on the homotype spindle. 
The pairing of the threads in these presynaptic pha'ses is therefore 
homologous with the pairing of the ttu’eads in the somatic prophases. 
Reference has already been made to the fact that it matters little 
whether the association of the univalent Segments in the postsynaptic 
phases be end to end, or in parallel series. Telosynaptists attach particular 
importance to the second contraction, during which, as first observed 
by Farmer and Moore (11), the pairing of univalent Segments is com- 
pleted. On the other hand, parasynaptists regard the whole series of post- 
synaptic phases as equally concerned in the fuLfilment of the pairing of 
univalent chromosomes inaugurated during the presynaptic prophases. 
Again, in Crepis virens, the evolution of the heterotype chromosomes, 
notwithstanding their low number, is not easy to foUow on account of the 
extremely viscous character of the nuclear contents. Many of the lengths 
of spireme, as they come out of synapsis, can be recognised as bivalent 
in nature, whilst other lengths afford no clue as to their character. The 
association of the univalent segments of the bivalent Strands may be 
complete, showing no trace of Separation, or the association may be loose, 
the univalent segments being widely parted along lengths of their course. 
There is no typical open spireme stage, the spü’eme passes ahnost im- 
mediately into second contraction. In the phases centring round second 
contraction the univalent spirenies may be associated end to end, or side 
by side, or in many intermediate ways. As the associated univalent seg- 
ments come out of second contraction they concentrate, and each pair 
becomes a heterotype chromosome. By closely following the sequence 
of events it can be ascertained that, the univalent segment of spireme, 
which emerges from synapsis, is homologous with the univalent segment 
which issues from second contraction, and this has then but to concen- 
trate to become the univalent segment of the bivalent chromosome. 
The study of the heterotype phase of Crepis virens suggests that 
during synapsis, not only a sorting out of univalent segments takes place, 
but also an association in pairs of the said univalent segments, an associa- 
tion which is completed during second contraction. 
Summary. 
1. Definite chromatic bodies have been found in the resting nuclei 
of the tapetum, in the resting nuclei between the last archesporial division 
and the heterotype prophase, and in the resting tetrad nuclei. Their 
