246 Rickett.—Fertilization in Sphaerocarpos. 
are visible, of which two are thicker and less regular in outline than the 
others, and may each represent two chromosomes close together. Many of 
the chromosomes are split, the split being visible through only a part of the 
length of each chromosome. The large X-chromosome is plainly visible. 
One chromosome is very small, and, owing to the fact that it has divided, 
almost ring-shaped ; this may be the Y-chromosome contributed by the 
male parent. 
The most important question suggested by a consideration of these 
stages and of those immediately preceding them is : Do the sexual nuclei 
actually fuse, after the chromosomes have formed, but while their membranes 
are still intact; or do the membranes break down simultaneously, leaving 
the chromosomes free in the cytoplasm ? On this point I have no positive 
evidence. No stages were found suggesting that fusion was taking place. When 
any nuclear membranes were present at all, the boundary separating the two 
nuclei (now always in contact) was always sharp and clear. The next stage 
found showed the chromosomes in one group, with nothing to indicate which 
were of paternal and which of maternal origin (except in the cases of the 
X- and Y-chromosomes), and surrounded by no membrane at all. If aji 
actual fusion of the nuclei occurs, it must be very rapid, and the fusion nucleus 
must lose its membrane almost immediately. It seems more probable that 
there is no actual fusion, but that each nuclear membrane disappears inde¬ 
pendently of the other, though perhaps at the same time. In this connexion 
it is interesting to note that Miss Black ( 1913 ) reports that the nucleus of 
the fertilized egg of Riccia probably undergoes a period of rest, during which 
the chromatin is collected into a densely staining cord or series of segments, 
while there may be less deeply staining and more finely divided material 
scattered throughout the rest of the nucleus. This description at once 
suggests my cases, during Phase 5, in which the female nucleus is under¬ 
going the first steps of spireme formation. Evidently Riccia differs from 
Sphaerocarpos in that its male nucleus has at this stage already fused with 
the female nucleus. The similarity is rather striking, and there is a possi¬ 
bility that, in Riccia as in Sphaerocarpos , the male nucleus is still separate 
from the female at this time, but was overlooked. 
Seventh Phase. 
This phase includes the anaphases and telophases of the first nuclear 
division of the zygote and the division of the latter into two cells. The 
wall formed in this division is always transverse, as in all other Liverworts 
(except Anthoceros) in which this division has been described. 
The daughter nuclei during the telophases are much smaller than either 
of the sexual nuclei just before the division. Because of the degree to which 
their chromatin is massed together, I could not follow the details of recon¬ 
struction. The daughter nuclei shortly pass into a resting state, and then 
