Nuclear Division in the Hepaticae. 515 
account for the clearance of the granules. But I think that 
any one who is familiar with the whole appearance presented 
by the spindle at the various stages of its existence in such 
a cell as, for example, the pollen mother-cell of a Lily, would 
admit that this will hardly supply a complete explanation of 
the appearances observed. 
The general conclusion to which I have arrived is one 
which, so far as I understand him, is also held by Heidenhain, 
namely, that the achromatic spindle is the direct effect of 
stresses and strains acting in the cell from definite centres, that 
their arrangement obeys ordinary mechanical laws, and that 
those constituents of the cell, be they cytoplasmic or be they 
nuclear, which possess the physical consistency necessary for 
the purpose, may be requisitioned to take part in the forma- 
tion of the spindle-fibres. 
Finally, there remain the chromosomes themselves to be 
considered. A considerable amount of knowledge has been 
accumulated with respect to the structure and mode of origin 
of these bodies in plants, chiefly through the investigations of 
Strasburger and of Guignard. But it will have been already 
seen from the foregoing account that the earliest stages in the 
development of these bodies are not in all cases identical, nor 
do they conform to one type in the mode of their further 
evolution. This is especially true of the divisions in the 
spore-mother-cells, as contrasted with the mitoses which occur 
elsewhere in the same organism. 
The features which seem to be distinctive of this spore- 
mother-cell division, at least in plants, are, first, the early 
period at which the longitudinal fission of the chromosome 
takes place, and, secondly, in the very peculiar and highly 
characteristic shape of the chromosomes when they reach 
their mature form on the spindle. 
As regards the first of these two points, the liverworts, 
described above, do not quite agree with the process of 
chromosome-differentiation as seen in Lilies, in which most of 
the detailed observations in this process have been carried 
out. In these plants, and also, it would seem, in the Gymno- 
