MEMBRANE CHANGES DURING STIMULATION 361 



egg the space between the fertilization-membrane and 

 the egg surface contains a colloidal substance which is 

 apparently separated from the egg at fertilization.'^ 

 Lyon has also observed an increased loss of catalase at 

 this time.^ McClendon and Gray have shown that a 

 significant increase in electrical conductivity occurs in 

 the sea-urchin egg immediately after fertilization.^ 

 Increase in the rate of entrance of dyes, and apparently 

 also of toxic substances, has also been observed.^ In the 

 Arbacia egg the rate of exchange of water in hypertonic 

 or hypotonic media is increased several times as a result 

 of fertilization.^ A change in the protoplasmic surface 

 layer, associated with increased permeabiKty to w^ater 

 and water-borne substances, thus appears to be a very 

 general accompaniment of both normal and artificial 

 activation. These facts, taken as a whole, suggest that 

 the first stage of the activation-process consists in a 

 breakdown, followed immediately by a re-formation, of 

 the external protoplasmic layer or plasma membrane. 



Recent observations by Just^ have emphasized still 

 more fully the resemblance between the primary or 

 surface change in the activation of egg cells and in the 

 stimulation of irritable tissues. In the large egg of the 

 sand-dollar, Echinarachnius (ca. 140 ju in diameter), 



^ Cf. Loeb, Parthenogenesis and Fertilization y^chdc^. xx, p. 208. 



2 Lyon, American Journal of Physiology, XXV (1909), 199. 



3 McClendon, American Journal of Physiology, XXVII (1910), 240; 

 J. Gray, Jour. Mar. Biol. Assoc, X (19 13), 50. 



4 McClendon, loc. cit.; Lyon and Shackell, Science, XXXII (1910), 

 249; Harvey, Science, XXXII (1910), 565. 



s American Journal of Physiology, XL (1916), 249. 



6 Just, ihid., LX (1922), 516. 



