24 Overton. — On the Organization of the Nuclei in the 
distributed throughout it. There is rather a framework, probably of linin, 
through which run two heterogeneous spirems, arranged parallel. 
By examining Figs. 3, 4, and 5, PI. I, it may be seen that the two 
prochromosome series and linin intervals of Thalictrum purpurascens are 
never widely separated from each other before synapsis. As already 
described, the prochromosomes are arranged in pairs at the period of 
greatest chromatic distribution, with every indication of parallel linin intervals. 
In all the earlier stages which I have studied these bodies and their linin 
connexions are apparently arranged in two spirems. This pairing of the 
prochromosomes and the linin intervals may possibly occur in the telophases 
of the last pre-meiotic division, so that they are already paired when the 
mother-cell nucleus is formed. The so-called leptonema or zygonema stage 
of the nucleus, therefore, in Thalictrum purpurascens does not originate by the 
transformation of a reticulum. The leptonema spirems are already present 
in the nucleus at the period of its greatest chromatic distribution. Just 
previous to the time when the synaptic contraction first becomes evident, 
the substance of the prochromosomes often spreads out along the linin 
intervals (Figs. 6, 7, and 8, PI. I). 
The first indication of the synaptic contraction in Thalictrum purpur- 
ascens consists in a concentration of the paired spirem threads either in the 
centre or at one side of the nuclear cavity (Figs. 6, 7, 8, PL I), so that the 
nucleus appears much more open and clear than at earlier stages. During 
the earlier stages of this contraction the threads, which constitute the frame- 
work, are present (Figs. 6, 7, PI. I), but these later disappear as the whole 
mass rounds up (Figs. 8, 9, PL I). Fig. 9, PL I, is an exceedingly highly 
magnified figure of an early phase of the contraction. In my former paper 
I have described the phenomenon of synapsis, which I held to be a normal 
process, in which a union in pairs of homologous elements is brought about, 
with a consequent pseudo-reduction. Although synapsis was described as 
a phase, during which a mutual interchange or interaction of parental 
elements might occur, I also expressed the opinion that the homologous 
chromosomes retained their identity during the process. A continuous 
bivalent linin spirem is thus produced, in which the bivalent chromosomes 
are present at more or less regular intervals. The paired chromosomes and 
linin threads are very closely associated, yet each prochromosome of each’ 
pair is distinctly visible except when one lies above the other. Fine linin 
threads run to the periphery of the nucleus apparently suspending the 
synaptic mass in the nuclear cavity. As this is only a section of the nucleus 
the nucleole is not present. Figs. 10 and 11, Pl. I, show complete contrac- 
tion stages, in which the prochromosomes, as well as the linin threads, are 
still evidently in pairs. The nucleole may or may not be entangled in the 
mass, as these figures show. Fine beaded linin threads run from the 
contraction mass to the periphery of the nucleus. Whether or not they are 
