I ce eS Oost Ee UR Sy Ra 
Be 
1995] MOORE—SPOROGENESIS IN PALLAVICINIA 93 
as tetrads just before the first mitosis. The achromatic structure 
which corresponds to FARMER’s quadripolar spindle appears during 
the prophase of the first nuclear division, and is followed by clearly 
defined bipolar spindles of the two successive mitoses with no evidence 
of accompanying centrosomes. The events of sporogenesis in Palla- 
vicinia Lyellii present then no fundamental differences from those of 
other liverworts and higher plants, the chief peculiarity being the 
rapidity with which the second mitosis follows the first. 
SUMMARY. 
1. The resting nucleus is spherical in shape and centrally situated 
in the spore mother cell. The spore mother cell is deeply four- 
lobed at an early period in its history. 
2. During synapsis the nucleus, containing a large and conspicuous 
nucleolus and a contracted chromatic thread, enlarges and becomes 
irregularly lobed. 
3. There is a distinct spirem stage in which a clear cut linin thread 
bears deeply staining chromatin granules. The thread shortens 
and thickens and at the same time the granules become larger and 
less numerous. 
4. The first evidence of a double spirem is observed just previous 
to the segmentation of the thread. 
5. The spirem segments into eight tetrads, which may be in the 
form of rings, Xs, Ys, Ts, or irregular masses. 
6. While these changes are taking place within the nucleus, the 
membrane becomes strongly lobed. Frequently, though not always, 
the form of the nucleus is tetrahedral, the angles projecting into the - 
respective lobes of the spore mother cell. 
7. There is no direct evidence of centrosomes or centrospheres 
and the indirect evidence is against their presence. 
8. The lobing of the nucleus is due to amoeboid motion in 
response to nutritive stimuli. 
9. The achromatic spindle originates in kinoplasmic caps to 
which the nuclear membrane contributes material. 
10. The distribution of the chromatin is effected through bipolar 
spindles in two successive mitoses. 
11. There is no resting stage between the first and second mitoses. 
