Fig. 94. Sperm-cells of Decapod Crustacea (after Koltzoff). a, Inachus scorpio; b, Galathea squamifera; c, do. after maceration, to shew spiral fibrillae.
The sperm-cells of the Decapod crustacea exhibit various singular shapes. In the Crayfish they are flattened cells with stiff curved processes radiating outwards like a St Catherine’s wheel; in Inachus there are two such circles of stiff processes; in Galathea we have a still more complex form, with long and slightly twisted processes. In all these cases, just as in the case of the blood-corpuscle, the structure alters, and finally loses, its characteristic form when the nature or constitution (or as we may assume in particular—the density) of the surrounding medium is changed.
Here again, as in the blood-corpuscle, we have to do with a very important force, which we had not hitherto considered in this connection,—the force of osmosis, manifested under conditions similar to those of Pfeffer’s classical experiments on the plant-cell. The surface of the cell acts as a “semi-permeable membrane,” {274} permitting the passage of certain dissolved substances (or their “ions”) and including or excluding others; and thus rendering manifest and measurable the existence of a definite “osmotic pressure.” In the case of the sperm-cells of Inachus, certain quantitative experiments have been performed[321]. The sperm-cell exhibits its characteristic conformation while lying in the serous fluid of the animal’s body, in ordinary sea-water, or in a 5 per cent. solution of potassium nitrate; these three fluids being all “isotonic” with one another. As we alter the concentration of potassium nitrate, the cell assumes certain definite forms corresponding to definite concentrations of the salt; and, as a further and final proof that the phenomenon is entirely physical, it is found that other salts produce an identical effect when their concentration is proportionate to their molecular weight, and whatever identical effect is produced by various salts in their respective concentrations, a similarly identical effect is produced when these concentrations are doubled or otherwise proportionately changed[322].
Fig. 95. Sperm-cells of Inachus, as they appear in saline solutions of varying density. (After Koltzoff.)
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 |
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
