6 jo Graham. — Nuclear Division of Preissia commutata. 
Summary. 
1. In earliest prophase the nucleus elongates. The linin becomes con- 
spicuous and large chromatin masses appear. 
2. Strands of linin thicken and form a spireme comparable to the spireme 
in many seed plants. The masses of chromatin lose their irregular outline 
and become distributed on the linin in the form of chromomeres. 
3. As the spireme is forming, it contracts about the nucleolus. Fine 
linin threads extend from the contracted spireme to the nuclear membrane. 
4. As the spireme recovers from the contraction and moves away from 
the nucleolus, it is very uneven. Later its width is even. 
5. The spireme now segments to form chromosomes, while the nuclear 
membrane is intact and the nucleolus still persists. The haploid number of 
chromosomes is probably eight. 
6. During late telophase the chromosomes elongate. They then become 
crowded so closely together that it is difficult to distinguish the individual 
ones, after which they elongate into slender threads which finally lose their 
individuality as they unite to form the reticulum of the daughter nuclei. 
7. The nucleolus is very large. It persists for some time after the 
segmentation of the spireme. Its disappearance is very sudden and coincident 
with the formation of the central spindle fibres. Neither vacuolization nor 
fragmentation of the nucleolus was observed at any time. This suggests 
that it may contribute some of its substance to the formation of the central 
spindle fibres. 
8. Granular masses of cytoplasm are applied to the elongated ends of 
the nucleus at early prophase. Between the granular cap and the nuclear 
membrane a cone-shaped cap rests on the ends of the nucleus. Fibres 
appear in these hyaline caps when the spireme is segmenting and the nuclear 
membrane is intact. The development of fibres in the hyaline cap in Preissia 
commutata is like their development in other groups of plants. 
9. Neither centrospheres nor centrosomes were found. 
II. Sporogenesis. 
(Plate LV.) 
The material for the study of the development of the spores of Preissia 
commutata was collected every day, beginning with April 18, until May 17, 
1912. In December the young sporogonium is fully formed. The stalk is 
short and lifts the receptacle but very little above the dorsal surface of the 
thallus. At this time the cells in the sporogonium form a solid tissue, and 
they remain in this condition until the following spring. Division in the 
sporogenous tissue begins early in April, but there is no differentiation of 
the tissue until May 1. On May 6, 1912, the sporogenous tissue was 
