230 BOTANICAL GAZETTE [ocrorer 
seem to be more definitely oriented with reference to the dis- 
tribution of the nuclei. At the stage of embryonic growth, when 
they are found in Pilobolus also, cell division and nuclear division 
are proceeding in a somewhat definitely correlated fashion, It 
can hardly be questioned that whereas in earlier stages the 
cleavage was largely independent of the nuclei, it comes later to 
be directed solely with reference to their distribution, and it 
seems not unnatural to assume that in this latter stage the 
nuclei control the orientation ot the cleavage planes. If this is 
the case, it is quite possible that the formation of the hyaline 
zones is the visible expression of this activity of the nuclei. 
On the other hand, it is quite possible to assume that cleav- 
age throughout is controlled by the cytoplasm, at first with little 
reference to the distribution of the nuclei, but later with special 
reference to the formation of uninucleated cells. The formation 
of hyaline zones preceding the cleavage furrows might in this case 
also mark the transition from the earlier irregular to the later 
more definitive Stage of cleavage without implying any special 
activity of the nuclei, I have already noted that it is quite as 
easy to assume that the cytoplasm itself contracts about the 
nuclei as that it is drawn together by a tension exerted from the 
nuclei. Either view is consistent with the assumption that mate 
rial for the growth of the plasma membranes is formed in the 
nucleus and passes outward from it to the newly-forming cel 
boundaries. As is seen from jigs. 6-9, the nuclei are dividing 
while the cleavage just described is going on, so that the unin 
cleated Segments formed become almost immediately binucleated- ; 
Cell division then follows either before or after the compl j 
Teconstruction of the daughter nuclei (figs. 26, 17): aie a 
the end, uninucleated spores are produced. The formation ‘ 
hyaline region and constriction furrow for the division of — : 
cleated cell whose nuclei are already in the anaphase nt 4 
shown in fig. 9. The beginning of the constriction for the # c 
division of a binucleated cell to form two uninucleated ee : 
the hyaline region having not yet appeared, is shown” - 
226. S 
