14 The First Lines of Morphology 
form the surface in contact with the medium is a minimum. 
Thus, ¢., they will be lengthened into spindle-shapes (see ele- 
ments of many woods and muscles) ; for in such forms, when 
of the same volume as spheres, there is a larger surface, both 
external and internal, to enjoy the favourable conditions of 
existence. ad. The spindle may become a tubule (as in many 
algee and fungi, pollen on stigma, &c.) e. The lengthening cell 
may be spun out into a filament or fibre so small as to have 
no interior, and thus escape again into the region of invisi-~ 
bility, after having been already in it (Oscillatoriacex, &c.) 
f. But all these lengthened forms, when not prevented by — 
attachments, must ever tend to reproduce the spherical, and 
therefore to turn round upon themselves, giving to nature 
twisted forms and spirals in abundance (found even among 
the simplest Alge, Spirogyra, &c.) (2.) The cell, under con- 
ditions of existence still favourable, may also extend its sur- 
face to the benignant influence of the ambient medium by 
depressing itself in the line of some one axis, and thus be- 
coming, g, lenticular (as in blood-cells, &ec.), or h, discoid (as 
in epithelial cells, &.), or &, stellate or radiated, that is, discoid 
at the centre, filamentary at the margin (as in pith of rush, 
&c.); J, the spiral tendency showing itself in the filamentary 
part (as in antherozoids, &.) In short, the ways in which a 
cell may vary its form so as to embrace more fully the ambient 
medium, when the condition of that medium is favourable to 
the deployment of life, are almost endless. 
Epithelium. 
The same phenomenon—an opening of cells to the ambient 
medium—must also tend to ensue when, instead of a single 
cell, or a linear series of cells, there is a cellular mass. The 
interior of such a mass is, in fact, ina great measure secluded 
from the ambient medium altogether. It is only after perme- 
ation of the outer cells that the ambient medium can reach 
the inner cells. If, then, a single cell tend to open into an 
annulus or element of a tubule, so as to give ingress into its 
interior to the ambient medium when that medium invites to 
the deployment of life, much more will the cells forming the 
periphery of a cellular mass tend to do so. In order to pro- 
tect them, therefore, from this opening power of the medium, 
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