Vapour Pressure Changes in the Frog ' S Egg at Fertilization

INTRODUCTION The suggestion has been made that the response of an egg to the action of a spermatozoon or parthenogenetic agent may be similar to that of a nerve to stimulation (Lillie, 1909). If this were so, it might be expected that activation, whether by a parthenogenetic agent or by a spermatozoon, would cause an increase in the permeability of the egg surface; although this increase in permeability is transient in the case of stimulated nerve, muscle, or Nitella cells, it need not necessarily be so. On the other hand, though the plasma membrane of the sea-urchin egg is known to become more permeable to sparingly ionized substances after fertilization (Lillie, 1917), there is no evidence that it is more permeable to ions, as would be expected if activation were strictly analogous to the stimulation of nerve or muscle. There is, in fact, some evidence to the contrary; the most recent investigations make it doubtful if any decrease in membrane resistance after fertilization has ever been measured in sea-urchin eggs (Cole, 1928). In the frog's egg, which, like that of the sea-urchin, has been studied intensively, profound changes in osmotic pressure were said by Backmann & Runnstrom (1912, p. 344) to take place after fertilization: ' Durch die Befruchtung wird in der Eizelle von Rana temporaria eine erhebliche Reduktion des osmotischen Druckes des Ei-inhaltes hervorgerufen. Der osmotische Druck wird etwa bis zu einem Zehntel des osmotischen Druckes beim erwachsenen Frosch reduziert. Es herrscht Isotonie zwischen dem eben befruchteten Froschei und dem umgebenden Wasser.'* Their results are displayed in Table 1. From these it would seem that the fall