XI. The electrical conductivity of echinoderm eggs, and its bearing on the problems of fertilisation and artificial parthenogenesis

Abstract

The precise nature of the change which ushers in the development of the egg has been much discussed. Broadly speaking, the problem may be said to have been considered from two points of view, the chemical and the physical. According to the point of view, the spermatozoon or the parthenogenetic agent has been supposed to start a chemical change within or at the surface of the cell, or to alter the physical properties of the surface. Loeb, for instance, in a recently published book, ascribes development to the introduction of certain substances into the interior of the egg. Lillie, and McLendon, on the other hand, regard an increase in the permeability of the egg surface to electrolytes as the precursor of development. The experiments described in this paper favour in general the latter hypothesis; but as they do not support the theories of Lillie and McLendon in detail, it will be well to give at the outset a brief sketch of the work of these investigators. In 1910 Lillie showed that the unfertilised eggs of lose their pigment when placed in isotonic solutions of various sodium salts. The order of effectiveness of the anions was found to be Cl<Br<NO 3 <CNS<I; this is also the order in which these ions affect the state of aggregation of colloids. If the eggs are removed from these solutions after an appropriate interval, membranes are formed round the eggs and a certain number develop into larvae. In the following year he showed that the effect of these sodium salts can be inhibited by the addition of calcium salts to the solutions. Accepting the view of Hamburger, Höber, and others that the phenomena of cytolysis are primarily due to an increase in the permeability of the cell surface to ions, Lillie arrived at the following conception of the fundamental processes which attend the development of the unfertilised egg:— Before fertilisation the egg membrane is freely permeable to kations, but only very partially permeable to anions ; hence, the egg membrane must be the seat of an electrical charge which is determined by Nernst’s formula E = u‒v / u+v RT/F log c 2 / c 1 , where u is the velocity of the kation and v of the anion; RT/F is constant; c 2 and c 1 the concentration of electrolytes inside and outside the cell. After fertilisation, however, the value of v is increased, so that the surface polarisation is decreased. It is this loss of surface polarisation which is the fundamental factor in the develop­ment of the unfertilised egg. From various general conclusions Lillie concludes that no cell can withstand a sustained condition of increased permeability, for such a condition would disorganise the electrolytic contents of the cell. Hence, after fertilisation, the permeability must fairly quickly be reduced again to its former value. In the case of eggs which are treated with butyric acid Lillie holds that the subsequent treatment with hypertonic sea-water effects this decrease in permeability.