Pollen Mother -cells of Certain Plants . 31 
In Calycanthus floridus the prochromosomes vary in form and size in 
the pre-synaptic nuclei (Fig. 2, Pl. II). Fig. 9 a, PL II, shows that the pro- 
chromosomes are made up of granules apparently embedded in the linin sub- 
stance. These granules in the two components of a pair are different in form 
and size, and apparently bear no definite relation in position to each other. 
The fine linin threads which lead off from each prochromosome connect 
with corresponding threads in other parts of the nucleus. At this stage 
the two prochromosomes may be much more closely associated, so as to 
appear as almost fused (Fig. 9 b, PI. II). The granular structure may be 
invisible and the prochromosomes may appear as homogeneous structures, 
showing no trace of granules (Fig. 9 c, PL II). In this case the prochromo- 
somes are short, very dense bodies, resembling those of Thalictrum 
purpurascens. At this stage the prochromosomes may also be long, 
narrow structures (Fig. 9 d, PL II). In this case there is again agranular 
structure, with apparently similar groups of granules in the two prochromo- 
somes often lying opposite each other. 
Fig. ge, PL II, represents a condition found in very early synapsis, 
when the linin intervals of the spirem threads have somewhat shortened, 
bringing the chromatic portions closer together. There is an evident 
condensation of the chromatin, which continues until the prochromosomes 
appear all alike in size and shape and become homogeneous in late synapsis 
(Figs. 5 and 6, Plate II). Figs. 9 f and 9 g, PL II, show the form and 
structure of these bodies in late synapsis, which they also retain in the post- 
synaptic spirem (Figs. 7 and 8, PI. II). 
I have in a previous publication pointed out that the fine cross linin 
threads which connect the pre-synaptic elements gradually disappear in 
Calycanthus floridus , leaving two distinct parallel spirems, which pass 
gradually into the synaptic contraction (Fig. 3, Pl. II). These spirems 
approach each other very closely in the neighbourhood of the nucleole (Fig. 4, 
Pl. II). There is an apparent condensation or diminution of the chromatic 
substance of each prochromosome as this synaptic contraction proceeds 
(Figs. 3, 4, and 5, PL II), so that the stainable portion of each prochromo- 
some is much less when complete contraction is reached than at earlier 
stages. The prochromosomes at this stage show no very distinct granular 
structure, but appear as homogeneous bodies, staining uniformly black with 
haematoxylin or red with safranin (Fig. 6, PL II). Further details of the 
behaviour of the prochromosomes have already been fully described in my 
former paper, and the reader is referred to that study for a complete account 
of synapsis in this plant. 
The bivalent heterogeneous spirem, when it first begins to pass out ot 
synapsis and to be distributed in the nuclear cavity, appears in the form 
of loops, as in Thalictrum purpurascens , and as has been described for 
other forms generally in this stage (Fig. 6, Pl. II). These loops are not 
