Davis . — Cytological Studies on Oenothera . /. 
555 
Synapsis. 
The events of synapsis and the formation of the bivalent chromosomes 
are of all phases in the process of pollen formation very much the most 
difficult to understand. The changes of structure are subtle and hard 
to interpret, and the correct seriation of stages requires the greatest care. 
There is, moreover, in the nature of the synaptic processes, which are chiefly 
concerned with coiled threads, a degree of variation in the form and 
arrangement of the structures concerned that is frequently very puzzling. 
Great assistance, however, is afforded by the fact that at no time are all of 
the anthers of an Oenothera flower at exactly the same period of development, 
and the differences are sufficiently great to be very helpful in determining 
which of a series of synaptic stages are older and which younger. Indeed, 
there are at certain periods of synapsis marked differences between the 
stages of development in the four pollen chambers of the same anther, and 
even in different portions of the same pollen chamber. As is always true, 
as far as the writer is aware, the period of synapsis is very much longer 
than that of any other phase in the history of pollen and spore formation. 
The structure of the resting nucleus following the last mitosis in 
the archesporium is shown in Fig. 8, where it will be seen that the chromatic 
material is distributed as a number of deeply staining bodies around the 
periphery. The nuclei remain for some time in this condition. The first 
indication of approaching synapsis is the appearance of extremely delicate 
strands connecting these bodies (Fig. 9), and forming a very open network. 
The strands then rapidly thicken and become more numerous (Fig. 10) 
until finally the nucleus is filled with a relatively close reticulum (Fig. 11). 
During its development the chromatic bodies become smaller, apparently 
contributing their substance to the strands composing the network, and the 
nucleoli are left as the only large structures lying freely within the 
nucleus. The smaller nucleoli appear more conspicuous because there are 
no longer chromatic bodies with which they may be confused. 
The advent of synapsis is marked by the beginning of a general slow 
contraction of the reticulum away from the nuclear membrane, a contraction 
which carries most of the strands towards the centre of the nucleus (Fig. 12). 
During this process of contraction there is a marked change in the structure 
of the reticulum, which results in the differentiation of very numerous 
threads which are coiled in an intricate manner. These are at first united 
to one another at many points, but as synapsis proceeds the thread structure 
becomes more and more evident until it becomes clear that a spirem is to be 
developed. 
The synaptic contraction generally draws inward with it all of the 
smaller nucleoli which, together with the coiled and crowded threads, form 
a ball or knot so dense and compact that it is impossible to differentiate 
