234 KirKwoop: POLLEN-FORMATION IN CUCURBITACEAE 
At the conclusion of the synapsis period the chromatin appears 
in thread-like form. The longitudinal cleavage of the thread is 
quite plain before its transverse divisions are apparent. A consid- 
erable shrinkage of the chromatin now takes place and at the time 
just before the differentiation of the spindle, it has contracted to 
sixteen irregular masses which in some instances appear quad- 
ruple ; these portions representing the division of the chromatin 
giving rise to the chromosomes are very small and irregular in 
form. The chromosomes of MJicrampelis, by numerous counts, 
number sixteen in the pollen-mother-cell. 
In the earlier, presynaptic condition of the chromatin an inter- 
esting condition was observed. Instead of forming a single thread, 
a condition such as appears in FIGURE 33 is often apparent. The 
thread appears to be doubled and the chromatin-masses occur 
frequently in pairs. Overton” has described the same features in 
Thalictrum purpurascens, and Cardiff® finds it the regular condition 
in Acer platanoides and several other plants. In Zhalictrum the 
number of the chromatin-masses agrees with the number of somatic 
chromosomes and the inference is that these presynaptic masses of 
chromatin which enter synapsis in pairs are the chromosomes of 
the original mother-cells (archesporium). In Micrampelis the 
double thread in a presynaptic condition is not always easily 
demonstrated, but in certain cases appears distinctly. The chro- 
matin soon becomes massed around the nucleolus and synapsis 
results. In this condition the identity of the nuclear structures 1s 
entirely obscured, the chromatin forming a dense, darkly staining 
mass. It is difficult to believe that this appearance in preparations 
of beautifully fixed material is purely an artifact, and the opinion 
is gaining ground that the condition of synapsis represents the final 
fusion of hitherto only associated elements of different parentage, 
‘a view which agrees with other facts commonly observed during 
the division of the spore-mother-cell. Synapsis in the material 
here under consideration differs in no essential from the condition 
usually described, so far as could be determined, At the close of 
the period of synapsis the chromatin emerges in the form of a much 
coiled thread which shows a distinct longitudinal division (FIGURE 
37). A transverse segmentation is soon apparent (FIGURES 39) 
40, 41), and the chromosomes thus formed contract into short, 
