The Present Position of Cell-Wall Research. 163 
•especially in free cells ( e.g . pollen-grains), such a growth in thickness 
of the lamellae, often accompanied by remarkable differentiation of 
form and structure, does take place. Mention should here be made 
•of Schiitt’s work upon the centrifugal growth in thickness of the 
membranes of certain Peridineae and Diatomeje. This growth takes 
place by the agency of protoplasm which has flowed out from the 
interior of the cell through pores in the membrane and has reached 
the outer surface of the wall. One other question of importance 
remains to be considered before closing this note. It is that 
regarding the manner in which the differentiation of a cell-wall 
lamella is effected after it has once been formed by the protoplast, 
and, in some cases, become separated from the protoplast either 
by the interpolation of fresh lamellae or by the actual contraction 
of the protoplast itself. The remarkable observations of Fitting 
upon the macrospores of Isoetes and Selaginella have shown us a 
■case in which the protoplast is entirely separated from a membrane 
which still continues to grow and to differentiate. Fitting’s work 
has been confirmed by Denke and by Campbell. Moreover, I have 
some, as yet unpublished, observations upon the pollen-grains of 
certain Onagraceae in which I have found quite comparable pheno¬ 
mena. Both in sections of material fixed in various fluids and in 
living material examined in physiological salt or sugar solutions or 
in the plant’s own juice the protoplast was seen, at certain stages, 
to be contracted away from the growing and differentiating mem¬ 
brane. If these observations are a true record of what occurs in 
the normal cell—and the most critical examination of the pollen- 
grains has hitherto shown nothing to oppose this view—-it becomes 
an exceedingly difficult matter to offer an explanation of the manner 
in which the differentiation of such a membrane is effected. 
It is possible that the explanation must be sought for in the 
way in which the original molecules are deposited in the lamella 
when this is first formed in intimate attachment to, and under the 
direct agency of the protoplast. These molecules may, not impro¬ 
bably, be arranged after a definite and specific plan and they may 
be regarded as exerting an influence over the positions in which all 
■subsequent molecules are set, even after the directive agency of the 
protoplast has been removed. The process might be roughly com¬ 
pared to that of a complex crystallisation system in which the form 
and the position of the crystals is determined at the outset by the 
living element of the cell. Perhaps a deeper study of the finer 
structure of the lamellae, may help to throw some light on this 
