367 
Pollen Development in Lactuca. 
cells vary greatly in appearance, as shown in Figs. 2—4. Individual 
tapetal cells frequently break down even during presynaptic stages (see 
Fig. 1). They are at this time uninucleate. The tapetal cells become 
binucleate by a mitotic division at about the time of the onset of synizesis 
in the pollen mother-cells, or even a little earlier. This division may be 
somewhat delayed (Fig. 2), but the cells are usually all binucleate by the 
time synizesis is complete. The second mitotic nuclear division, which 
makes the tapetal cells quadrinucleate, occurs at the end of the period 
of synizesis or when the pollen mother-cell nuclei are just coming out 
of synizesis and forming a loose spireme. These nuclear divisions are 
usually simultaneous throughout a loculus. 
Among the numerous types of tapetal cells, many of them remain, 
at least for some time, in the binucleate condition. Occasionally, crowding 
of the spindles or failure of one nucleus to divide will result in a trinucleate 
cell. The variations and later history of these peculiar tapetal cells will be 
considered below. 
Synizesis and Synapsis. 
In this paper the term synizesis is adopted for the tightly contracted 
phase of the nucleus, following the usage which has become customary in the 
literature of animal cytology, while the term synapsis is applied to the whole 
period from the beginning of contraction in the nucleus, through the various 
stages of spireme formation until the spireme segments into chromosomes. 
Synizesis thus represents the height of the synaptic contraction, which 
takes place in the earlier stages of synapsis. The essential feature of 
synapsis has always been considered to be the pairing of elements of 
maternal and paternal origin. The more recent discovery that the maternal 
and paternal chromosomes frequently show a tendency to be paired in the 
somatic nuclei of many plants and animals has rendered it very doubtful 
whether synapsis is ever concerned at all, in organisms in which telosynaptic 
reduction takes place, in actually bringing about the paired association. 
This phase of the subject will be considered later. 
Fig. 5 represents a presynaptic pollen mother-cell in which the 
nucleus is occupied by a uniformly distributed reticulum of fine, anastomos- 
ing threads. This threadwork is only slightly thickened at the nodes, and 
there is no trace of prochromosomes in the resting stage. Almost invaria- 
bly a single large nucleolus is present, but very occasionally two have been 
observed. The first indication of the onset of synizesis is the appearance of 
a clear space at one side of the nucleus, accompanied by a thickening of the 
reticulum threads (Fig. 6). Many mother-cell nuclei have been seen in 
this condition where other nuclei in the same section show later stages of 
synizesis, as represented in Figs. 7-1 1. The cytoplasm of such cells is 
properly fixed and not unduly contracted. 
B b 2 
