Herbst had shown that when sea-urchin larvæ were raised in a medium in which only one of the constituents of the sea water was lacking (not only NaCl, KCl, or CaCl{ 2}, but also Na{ 2}SO{ 4}, NaHCO{ 3}, or Na{ 2}HPO{ 4}), the eggs could not develop into plutei; from which he concluded that every constituent of the sea water was necessary. This would indicate a case of extreme adaptation to all the minutiæ of the external medium.
Experiments on a much more favourable animal for this purpose, namely, the eggs of the marine fish _Fundulus_, gave altogether different results. The eggs of this marine fish develop naturally in sea water but they develop just as well in fresh or in distilled water, and the young fish when they are made to hatch in distilled water will continue to live in this medium. This proves that these eggs require none of the salts of the sea water for their development. When these eggs are put immediately after fertilization into a pure solution of NaCl of that concentration in which this salt exists in the sea water practically all the eggs die without forming an embryo; but if a small quantity of CaCl{ 2} is added every egg is able to form one, and these embryos will develop into fish and the latter will hatch. This led the writer to the conclusion that these fish (and perhaps marine animals in general) need the Ca of the sea water only to counteract the injurious effects which a pure NaCl solution has if it is present in too high a concentration.[261] When we raise the eggs in a pure NaCl solution of a concentration less than or equal to m/8 practically every egg will develop; and even in a m/4 or 3/8 m many or some eggs will form embryos without adding Ca; it may be that a trace of Ca present in the membrane of the egg may suffice to counter-balance the injurious action of a weak salt solution.
[261] Loeb, J., "The Poisonous Character of a Pure NaCl Solution." _Am. Jour. Physiol._, 1900, iii., 329; _Arch. f. d. ges. Physiol._, 1901, lxxxviii., 68; _Am. Jour. Physiol._, 1902, vi., 411; _Biochem. Zischr._, 1906, ii., 81.
The concentration of the NaCl in the sea water at Woods Hole (where these experiments were made) is about m/2, and as soon as this concentration of NaCl is reached the eggs are all killed as a rule before they can form an embryo, unless a small but definite amount of Ca is added. It was found that the eggs can be raised in much higher concentrations of NaCl, but in that case more Ca must be added. The following table gives the minimal amount of CaCl{ 2} which must be added in order to allow fifty per cent. of the eggs to form embryos. (The eggs were put into the solution an hour or two after fertilization.)