82 
THE ORGANIC CELL 
though doubts have been cast on it by the recent researches 
on some of the higher plants, in which the presence of a centro- 
some has so far defied demonstration. If the latter be true, 
the centrosome loses much of its former importance, and must 
be looked upon as playing only a subordinate part in the 
mechanism of mitosis. 
The Cell-membrane 
The envelope of the cell belongs to one of the passive or 
metaplastic products of protoplasm. 
As a rule, in animal-cells the walls are only very slightly 
developed ; among plants the peripheral envelope of the cell is 
of great importance, often attaining a great thickness. 
A notable exception to the extreme thinness of cellular 
envelopes in animal-cells may be mentioned : the intercellular 
matrix in cartilage. 
There is a great probability that all cells have to a certain 
extent a specially differentiated envelope. Even among leuco- 
cytes there is a differentiation of the peripheral protoplasm 
into a firmer layer, forming a kind of skin or pellicle. Recent 
research tends to show that cell-walls are generally produced 
by secretion, though there are cases in which the protoplasm 
of the cell itself is so altered at its surface as to form an envelope. 
Cell Polarity 
In a great number of cells there is a symmetrical arrange- 
ment of the parts in relation to an axis passing from pole to 
pole. The idea of polarity has been worked out along two 
different lines, one a morphological conception, the other a 
purely physiological one. 
Van Beneden working along the line of morphology 
conceived the organic axis as passing through the nucleus and 
centrosome. Heidenhain has elaborated this theory, teaching 
that all the structures of a cell have a fixed relation to the axis, 
going so far as to state that this relation is brought about 
by tension in the astral rays, the fixed point being at the 
centrosome. 
On the other hand, according to Rabl and Hatschek, cell- 
polarity 4 is a polar differentiation of the cell-substance arising 
