CHARLES B. METZ 37 



not induce breakdown of neighboring granules. Therefore some invisible mecha- 

 nism musL operate in the propagalive reaction. 



Two views concerning the mechanism of i)ropagation of the cortical responses 

 have been advanced. The first view considers the reaction to be a self-proi)a- 

 gating chain reaction. Such a chain reaction system was formulated by Lillie 

 (48) in his fertilizin theory. Lillie visualized a chain reaction involving combi- 

 nation of the fertilizin and antifertilizin of the egg, and it must be granted that 

 the possibility of some such reaction between fertilizin and antifertilizin has 

 not been ruled out. A number of other hypothetical schemes for a self-propa- 

 gating chain reaction may be formulated. For example, an essential proenzyme 

 might be distributed throughout the egg cortex or even the entire egg. If this 

 I^roenzyme were converted locally to the active form by the sperm and if the 

 enzyme autocatalytically convert more proenzyme into the active material, a 

 relatively simple chain reaction might result. The conversion of chymotryp- 

 sinogen to chymotrypsin by action of the latter (12) is one of a number of 

 examples of this type of proenzyme-enzyme relationship. In this connection 

 it is of interest that proteolytic enzymes are activated in the egg at fertilization 

 (53). Others (72, 97) have proposed somewhat similar but more complex mecha- 

 nisms for egg activation and propagation of the cortical response. 



Several investigators have considered the possibility that the passage of the 

 cortical response might be associated with the propagation of an action potential 

 similar to that of muscle and nerve. Rothschild and Swann (89) have reviewed 

 the earlier studies. Although neither a resting potential across the egg surface 

 nor an action potential at fertilization had been demonstrated in any egg 

 at that time, more recent studies have revealed both. Scheer el al. (104) recorded 

 transient egg potentials associated with fertilization. These investigators were 

 unable to detect a resting potential in the sea urchin egg. More recently, how- 

 ever, Grundfest^;/ al. (32) have demonstrated a resting potential of 40 60 milli- 

 volts across the surface of the Asterias egg, and Tyler el al. (125) have observed 

 a drop of 5-10 millivolt in this potential 15-30 seconds following insemination. 

 After about 30 seconds the potential rose to the resting level, and later surpassed 

 it. Evidently, then, the echinoderm egg does exhibit a resting potential and a 

 potential change occurs during the activation of the egg. Apparently the diffi- 

 culty in recording these phenomena in the past has resulted from failure of the 

 electrode to penetrate the plasma membrane of the egg (52, 124). A closer 

 correlation of the electrical changes with other activation phenomena of the 

 egg may shortly be expected to result from the efforts of these investigators. 



A second possible mechanism of propagation is the diffusion of some sub- 

 stance from the sperm. Rothschild (84) has emphasized that the rate of propa- 

 gation of the cortical reaction is not incompatible with such a mechanism. 

 Local reactions in the neighborhood of the penetrating sperm that might well 

 result from such diffusion have been demonstrated in nematode eggs by cyto- 



