Thus the following table shews the percentage concentrations of certain salts necessary to bring the cell into the forms _a_ and _c_ of Fig. 95; in each case the quantities are proportional to the molecular weights, and in each case twice the quantity is necessary to produce the effect of Fig. 95_c_ compared with that which gives rise to the all but spherical form of Fig. 95_a_. {275} % concentration of salts in which the sperm-cell of Inachus assumes the form of ───────────────────── fig. _a_ fig. _c_ Sodium chloride 0·6 1·2 Sodium nitrate 0·85 1·7 Potassium nitrate 1·0 2·0 Acetic acid 2·2 4·5 Cane sugar 5·0 10·0 [Illustration: Fig. 96. Sperm-cell of _Dromia_. (After Koltzoff.)] If we look then, upon the spherical form of the cell as its true condition of symmetry and of equilibrium, we see that what we call its normal appearance is just one of many intermediate phases of shrinkage, brought about by the abstraction of fluid from its interior as the result of an osmotic pressure greater outside than inside the cell, and where the shrinkage of _volume_ is not kept pace with by a contraction of the _surface-area_. In the case of the blood-corpuscle, the shrinkage is of no great amount, and the resulting deformation is symmetrical; such structural inequality as may be necessary to account for it need be but small. But in the case of the sperm-cells, we must have, and we actually do find, a somewhat complicated arrangement of more or less rigid or elastic structures in the wall of the cell, which like the wire framework in Plateau’s experiments, restrain and modify the forces acting on the drop. In one form of Plateau’s experiments, instead of supporting his drop on rings or frames of wire, he laid upon its surface one or more elastic coils; and then, on withdrawing oil from the centre of his globule, he saw its uniform shrinkage counteracted by the spiral springs, with the result that the centre of each elastic coil seemed to shoot out into a prominence. Just such spiral coils are figured (after Koltzoff) in Fig. 96; and they may be regarded as precisely akin to those local thickenings, spiral and other, to which we have already ascribed the cylindrical form of the Spirogyra cell. In all probability we must in like manner attribute the peculiar spiral and other forms, for instance of many Infusoria, to the {276} presence, among the multitudinous other differentiations of their protoplasmic substance, of such more or less elastic fibrillae, which play as it were the part of a microscopic skeleton[323]. ――――――――――