Page:Popular Science Monthly Volume 86.djvu/573

Rh sea-urchin eggs, which normally die if not fertilized. Morgan, in 1899, produced segmentation in unfertilized sea-urchin (Arbacia) eggs, by immersing them in sea water to which a dry salt had been added, such a solution being called hypertonic sea water, and having a greater osmotic pressure than ordinary sea water. Loeb, immediately afterwards, produced swimming larvæ in sea water made hypertonic by the addition of magnesium chloride. Since then many investigators have studied the subject. Eggs of various animals have been made parthenogenetic by putting them in solutions containing salts, acids or alkalis, sugar, fat solvents, blood sera, alkaloids, or by means of asphyxiation, or by mechanical, thermal or electric changes. The concentration of the solution in which the eggs are treated may be the same as that of the fluid in which they normally live, or it may be of a greater or less concentration.

It has been fairly well demonstrated that the artificial agents—used in producing parthenogenesis—act primarily on the surface of the egg, and R. Lillie supposes they tend to increase its permeability. Loeb recognizes a "superficial cytolysis," the exact nature of which is, however, unknown. When cells containing soluble coloring matters undergo cytolysis, the colored substances come out of the cells. Cytolysis has, therefore, been considered to consist of, or be accompanied by, an increase in permeability of the protoplasm.

The electric conductivity method of Kohlrausch suggested itself to the writer as the best way of settling this question of the permeability of the egg. The principle of the method lies in the fact that an electric current is carried through wet substances by the movement of electrically charged atoms or "ions." If the permeability increases, the ions move faster and the current is greater. The use of this method showed that the permeability increased immediately after fertilization, on the application of agents producing parthenogenesis. These results were confirmed by Gray, who observed further that the permeability decreased again about fifteen minutes after fertilization. Lyon and Shackell observed that the permeability of the egg to certain dyes increased on fertilization, and also that more of the red substances came out of the fertilized than out of the unfertilized eggs. E. N. Harvey observed independently that the permeability of the eggs to certain dyes and caustic soda increased on fertilization. Finally, Glaser has recently concluded from his experiments that fertilization increases the egg's permeability.

There is another proof that the egg of one species (the frog) becomes more permeable to salt on beginning development. The writer