426 BOTANICAL GAZETTE [JUNE 
are clearly distinct from the network. During synapsis the reticulum becomes 
transformed into a definite spirem. the fibers thickening apparently by the dis- 
tribution of material from the knots, which become less conspicuous. The 
fibers now arrange themselves in pairs, and a general contraction of the reticulum 
takes place, probably associated with this approximation of the fibers. There 
is thus formed a continuous spirem in the nucleus, which is plainly composed 
of two slender threads lying side by side and probably with no free ends. The 
two threads often run closely parallel, sometimes loosely twisted about one another, 
sometimes in contact and apparently fused, ti rather widely separ ated. 
It is clear that the double nature of the spirem is not due to a fission, but that 
two independent threads are developed out of the reticulum. The two threads 
are regarded as of paternal and maternal origin respectively. They gradually 
fuse, so that in later stages of synapsis the nucleus appears to contain a relatively 
thick single (fusion) spirem, which is shorter and more loosely coiled than in earlier 
stages. Each thread of the pair consists of a series of chromatin granules (chro- 
momeres) imbedded in a ground substance (linin). As the two threads unite, 
the chromomeres generally come together in pairs and fuse to form a single row 
of larger chromomeres. The single (fusion) spirem on emerging from synapsis 
becomes uniformly distributed throughout the nucleus as a much convoluted 
thread. Some of the loops become fastened to the periphery of the nucleus. 
There is no regularity in the number of chromatic segments which are formed 
later. 
While evenly distributed the single (fusion) spirem undergoes a longitudinal 
fusion into two threads, which is preceded by a division of each fusion chromomere. 
This is the first longitudinal fusion, well known from the descriptions of GREGOIRE, 
GuIGNARD, Mortier, and others. The split spirem now segments into the 
reduced number of chromatic elements (bivalent chromosomes) characteristic 
of the heterotypic mitosis. The first free ends of the segments usually appear 
at the periphery. of the nucleus, where it is evident that the split spirem breaks 
apart at the loops. The segments of the split spirem now shorten and thicken, 
taking on the various forms peculiar to the heterotypic mitosis. They are 
obviously pairs of chromosomes, which stand for the full number of sporophytic 
elements (24), now associated to form a reduced number (12) of bivalent chromo- . 
somes. Shortly after the segmentation of the spirem the chromosomes of each 
pair may show evidence of the second longitudinal fission, which is completed 
during the metaphase of the heterotypic mitosis. Meanwhile the spindle of the 
heterotypic mitosis is being organized, the position of the bivalent chromosomes 
is shifted, and they are arranged on the nuclear plate where the elements of each 
pair separate as the so-called daughter chromosomes of the heterotypic mitosis. 
This first division in the spore mother-cell then simply distributes in two sets. 
the twenty-four sporophytic chromosomes which ALLEN believes to have been 
derived from a maternal and paternal spirem. The second longitudinal fission 
which becomes conspicuous during metaphase of the heterotypic mitosis is clearly 
a premature fission of the chromosomes which are to enter the second nuclear 
