Acer negundo, Z., and Staphylea trifolia , Z. 
1 29 
Summary and Conclusions. 
The resting nucleus of both Acer negundo and Staphylea trifolia 
consists of a delicate linin network, or reticulum, upon which are distributed 
chromatin granules or collections of chromatin granules of varying sizes, and 
a large nucleolus. 
Before synapsis there is no chromatin spireme present, nor is there any 
structure referable to any definite spireme or spiremes. 
Synapsis is a normal phenomenon, and consists in the contraction of 
the nuclear reticulum into a more or less compact mass, which may or may 
not include the nucleolus. 
Within the synaptic mass, and during the loosening up of the same, 
the definite chromatin spireme is formed. The assembling of the granules 
of the nuclear network into the contracted mass probably represents the 
initiatory step or steps in the formation of the spireme. The spireme is 
formed by the serial arrangement of the chromatin granules or groups of 
granules into a row of chromomeres, and furthermore by the union of certain 
meshes of the network along with the severing of others. This seems 
to be the only way a continuous thread of spireme can be formed from 
a network. 
In Acer negundo the spireme appearing immediately on the loosening 
up of the synaptic contraction does not show a longitudinal split, which, 
however, may be observed in Staphylea at this and at later stages. Taking 
all of the pollen mother-cells into consideration, the appearance of the longi- 
tudinal split is probably the exception rather than the rule even in Staphylea . 
The view accepted here is that the spireme has undergone a longitudinal 
fission. If the chromomeres do not split lengthwise, they must be formed 
or assembled into the spireme as a double row instead of as a single row of 
granules ; for there is no fusion of two spiremes, either before, or during, or 
after synapsis. In the opinion of the writer, the formation of the spireme 
is more probably the cause of synapsis than that synapsis is the cause of 
the formation of the spireme. 
In both species there is finally developed the uniform hollow spireme, 
which is either endless, or, if free ends are present, these are few in number. 
This spireme consists of the diploid number of chromosomes placed end 
to end. 
In both, there is present the feature known as the second contraction, 
or rearrangement of the spireme from the more regular to a more compli- 
cated form, in which are to be seen knotted and entangled parts, from which 
may extend a varying number of straight portions or loops. The looping 
is more frequent in Staphylea than in Acer negundo , where radiating loops 
from an entanglement may only rarely occur. There is not the regularity 
K 
