THEORIES OF FERTILIZATION 271 



Lillie, R. S., '41. Like Loeb, R. S. Lillie conceived of cortical changes as 

 being the main aspect of activation, particularly changes such as a decrease 

 in viscosity which permits interaction of various substances which normally 

 are kept separated in the unactivated egg. Lillie's hypothesis may be stated 

 as follows: An activating substance, comparable to a growth hormone or 

 auxin, is formed in the egg. This substance may be called (A). The forma- 

 tion of (A) results from the interaction of two substances, (S) and (B), 

 present in low concentrations in the egg. One of these substances, (S), is 

 synthesized in the egg by treating the egg in various ways, such as immersion 

 in sea water in the presence of oxygen. The other substance, (B), is freed 

 from pre-existing combination by a simple splitting (hydrolytic) process ini- 

 tiated or catalyzed by acid. This reaction is independent of oxygen. The 

 union of the two substances, (S) and (B), forms the activating substance, 

 (A). Lillie thus believes in a single factor as the initiator of development. 

 Complete activation of the egg results when (A) is produced in adequate 

 concentration; partial activation occurs when it is present in quantity below 

 the optimum concentration. 



For discussion, see Brachet, J., '50, p. 141. 



Heilbrunn, L. V., '15, '28, '43. This author believes that an increase in 

 viscosity with resultant coagulation or gelation of egg cytoplasm is involved 

 directly with the initiation of development. Heilbrunn regards this gelation 

 process to be similar to the clotting of blood. He also regards calcium as 

 the main agent in bringing about this effect, and therefore believes calcium 

 to be concerned directly with egg activation. According to this view, calcium 

 is bound to the proteins localized in the egg cortex. At the time of activation, 

 artificially or by sperm contact, part of this calcium is liberated which in turn 

 produces a coagulation of the cytoplasm, initiating development. Dalcq and 

 his associates also have emphasized the importance of calcium in the acti- 

 vation process. 



For discussion, see Brachet, J., '50, p. 146; Dalcq, '28; Runnstrom, '49. 



Runnstrom, J., '49. Runnstrom more recently has contended that an in- 

 hibitor of proteolytic enzymes may be present in the vitelline membrane and 

 cortex of the egg. He assumes that the inhibitor, possibly fertilizin, may be 

 identical with a heparin-like substance. He further assumes that the inhibitor 

 is bound to a kinase and is released when protein substances associated with 

 the sperm unite with the inhibitor. "A kinase acting on a proenzyme may then 

 be released"; the latter, i.e., the kinase, acts upon the proenzyme in the cortex 

 of the egg, giving origin to an enzyme or enzymes which initiate development. 

 Runnstrom's position in essence is a modern statement of the inhibition theory 

 of F. R. Lillie (see J. Morphol., vol. 22). 



