568 Davis.— Cytological Studies on Oenothera. I. 
strands towards the centre of the nucleus. During the process of contrac- 
tion numerous threads are differentiated from the reticulum which become 
coiled in a very intricate manner. 
6. The synaptic contraction draws the coils of threads into a dense 
knot close to the large nucleolus, which generally lies at one side of the 
nucleus. Loops of the threads extend into the nuclear cavity from the 
synaptic knot as a centre. The threads gradually thicken as synapsis 
proceeds, and the length of the thread system is very much shortened. 
7. When fully contracted, the synaptic knot consists of much-thickened 
threads (constituting the spirem) which are drawn so tightly together that 
their arrangement cannot be traced. The loops extending from the con- 
tracted mass are at this stage thick and very conspicuous. 
8. The contracted material emerges from synapsis by a general 
loosening up of the elements of the contracted knot. It then becomes at 
once apparent that the chromatin is in the form of a group of seven rings, 
some of which are derived from the loops referred to in paragraph 7. The 
rings are generally closed, but consist of semicircular halves, which are the 
fourteen chromosomes. The rings are therefore bivalent chromosomes, 
very characteristic structures of diakinesis and the prophases of the 
heterotypic mitosis. 
9. The seven ring-shaped bivalent chromosomes become much con- 
densed during the prophases of the heterotypic mitosis. They then become 
arranged at metaphase to form the equatorial plate, after which the halves 
separate as two sets of V-shaped sporophytic chromosomes (seven in each 
set) that pass to the poles of the spindle. The heterotypic mitosis is there- 
fore a reduction division. A lengthwise fission of each chromosome which 
takes place during anaphase is the premature division of these elements in 
preparation for the succeeding homotypic mitosis. 
10. The sporophytic chromosomes of the heterotypic mitosis are 
V-shaped and appear essentially similar to one another. There seems to 
be as great uniformity in their shape and size as is common for the hetero- 
typic mitoses of plants. 
11. A resting nucleus is organized between the heterotypic and 
homotypic mitoses, and throughout this period of interkinesis the seven 
split chromosomes may be readily followed. 
12. The homotypic mitosis distributes the halves of the split chromo- 
somes into two sets (seven in each), and is consequently an equation 
division. 
Woods Hole, Massachusetts. 
April , 1909. 
