Why was it not possible to prove this fact for the eggs of the sea urchin? Before we answer this question, we wish to enter upon the discussion of the nature of the injurious action of a pure NaCl solution of a certain concentration and of the annihilation of this action by the addition of a small quantity of Ca. The writer suggested in 1905 that the injurious action of a pure NaCl solution consisted in rendering the membrane of the egg permeable for NaCl, whereby the germ inside the membrane is killed; while the addition of a small amount of Ca (or any other bivalent metal) prevents the diffusion of Na into the egg,[265] possibly, as T. B. Robertson[266] suggested, by forming a precipitate with some constituent of the membrane, whereby the latter becomes more impermeable. The correctness of this idea can be demonstrated in the following way. When eggs of Fundulus, which are three or four days old and contain an embryo, are put into a test-tube containing 3 m NaCl they will float on this solution for about three or four hours; after that they will sink to the bottom. Before this happens the egg will shrink and when it ceases to float the embryo is usually dead. This is intelligible on the assumption that the NaCl solution entered the egg, increased its specific gravity so that it could not float any longer and killed the embryo. When we add, however, 1 c.c. 10⁄8 m CaCl2 to 50 c.c. 3 m NaCl the eggs will float, the heart will continue to beat normally and the embryo will continue to develop for three days or more, because the calcium prevents the NaCl from entering into the egg.[267] For if we put a newly hatched embryo into 50 c.c. NaCl+1 c.c. 10⁄8 m CaCl2 it will die almost instantly; hence the membrane must have acted for three or more days as a shield which prevented the NaCl from diffusing into the egg in the presence of CaCl2.
The same experiments cannot be demonstrated in the sea-urchin egg, first, because it can live neither in distilled water nor in very dilute nor very concentrated solutions; and second, because it is not separated as is the germ of the Fundulus egg from the surrounding solution by a membrane which is under proper conditions practically impermeable for water and salts.
Nevertheless it can be shown that the results at which we arrived in our experiments on Fundulus are of a general application. Osterhout[268] has shown that plants which grow in the soil or in fresh water are readily killed by a pure NaCl solution of a certain concentration, while they can resist the same concentration of NaCl if some CaCl2 is added. Wo. Ostwald[269] has shown the same for a species of Daphnia. We, therefore, come to the conclusion that the injurious action following an alteration in the constitution of the sea water is in some of the cases due to an increase in the permeability of the membranes of the cell, whereby substances can diffuse into the cell which when the proper balance prevails cannot diffuse. For this balance the ratio of the concentration of the salts with univalent cation Na and K over those with bivalent cation Ca and Mg CNa+K salts/CCa+Mg salts is of the greatest importance.
6. The importance of this quotient appears in the so-called “behaviour” of marine animals. We have mentioned the newly hatched larvæ of the barnacle in connection with heliotropism. These larvæ swim in a trough of normal sea water at the surface, being either strongly positively or negatively heliotropic. They collect as a rule in two dense clusters, one at the window and one at the room side of the dish. If such animals are put into a solution of NaCl+KCl (in the proportion in which these salts exist in the sea water), they will fall to the bottom unable to rise to the surface. They will, however, rise to the surface and swim energetically to or from the window if a certain quantity of any of the chlorides of a bivalent metal, Mg, Ca, or Sr, is added, but these movements will last only a few minutes when only one of these three salts is added; and then the animals will fall to the bottom again. If, however, two salts, e. g., MgCl2 and CaCl2, are added the animals will stay permanently at the surface and react to light as they would have done in normal sea water. These animals also can resist comparatively large changes in the concentration of the sea water, and it seemed of interest to find out whether the quotient CNaCl+KCl/CMgCl2+CaCl2, which just allowed all the animals to swim at the surface, had a constant value. The MgCl2+CaCl2 solution was 3⁄8 m and contained the two metals in the proportion in which they exist in the sea water; namely, 11.8 molecules MgCl2 to 1.5 molecules CaCl2. The next table gives the result.[270] Since these experiments lasted a day or more each, usually two different concentrations of NaCl+KCl of the ratio 1 : 2 or 1 : 4 were compared in one experiment.
TABLE XVIII
These experiments indicate that the ratio of CNa+K/CCa+Mg remains very nearly constant with varying concentrations of CNa+K.
In former experiments on jellyfish the writer had shown that there exists an antagonism between Mg and Ca[271], and this observation was subsequently confirmed by Meltzer and Auer[272] for mammals. It was observed that in a solution of NaCl+KCl+MgCl2 the larvæ of the barnacle were also not able to remain at the surface for more than a few minutes, while an addition of some CaCl2 made them swim permanently at the surface. Various quantities of MgCl2 were added to a mixture of m/4 or m/2 NaCl+KCl, to find out how much CaCl2, was required to allow them to swim permanently at the surface.
TABLE XIX
| C.c. of m/16 CaCl2 Necessary to Induce the Majority of the Larvæ to Swim in | ||||
|---|---|---|---|---|
| m/2 (Na+K) | m/4 (Na+K) | |||
| 50 c.c. NaCl+KCl+0.75 c.c. 3⁄8 m MgCl2 | 0. | 2 | ||
| 50 c.c. NaCl+KCl+ 1.5 c.c. 3⁄8 m MgCl2 | 0. | 4 | 0. | 3 |
| 50 c.c. NaCl+KCl+ 2.5 c.c. 3⁄8 m MgCl2 | 0. | 4 | 0. | 4 |
| 50 c.c. NaCl+KCl+ 5.0 c.c. 3⁄8 m MgCl2 | 0.7– | 0.8 | 0.7– | 0.8 |
| 50 c.c. NaCl+KCl+10.0 c.c. 3⁄8 m MgCl2 | 1. | 6 | 1. | 6 |
| 50 c.c. NaCl+KCl+15.0 c.c. 3⁄8 m MgCl2 | 1. | 8 | ||
| 50 c.c. NaCl+KCl+20.0 c.c. 3⁄8 m MgCl2 | 1. | 8 | ||